It’s possible for two distinct languages to be mutually intelligible. It does not follow that they are “merely” dialects of the same language.
In most cases, though, it either requires prolongued contact, so that most people who speak one “sorta-kinda” speak the other, or that both parties are making a special effort to be understood, and maybe some special concessions are happening.
Few examples: Czech and Slovak are mutually intelligible. They are distinct linguistically, but much vocabulary is shared, both very old vocabulary from a shared history, but also recent vocabulary adopted as loan words during the time of “Czechoslovakia.” Not to mention, there was lots of contact, and people moving from one area to another, so people like an aunt of mine, Slovak by birth, but living in Prague since he 20s, and married to a Czech, chose Czech citizenship upon the “Velvet Divorce.”
Other examples: American Sign Language is an offshoot of French Sign Language, and they retain a degree of mutual intelligibility, provided the speakers wish to be understood. They also use the same manual alphabet, so if either party understands the written language of the other, a lot of confusion can easily be cleared up.
I did not say that they are merely dialects of the same language. I would never use the word “merely”, among other things. I said that there are cases where two varieties (where “variety” means something that can be either a language or a dialect or something in between) are closer than being two different languages and further apart than being two different dialects. There are also cases like the following:
There are ten villages in a straight row in a particular region.
Village 1 is 20 miles from village 2. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 2 is 20 miles from village 3. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 3 is 20 miles from village 4. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 4 is 20 miles from village 5. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 5 is 20 miles from village 6. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 6 is 20 miles from village 7. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 7 is 20 miles from village 8. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 8 is 20 miles from village 9. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 9 is 20 miles from village 10. The variety spoken in one of them is close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as just two dialects of a single language.
Village 1 is 180 miles from village 10. The variety spoken in one of them is not close enough to the other the variety spoken in the other one that they can understand each other. If they were the only two villages in this row of ten villages, you would think of them as two different languages.
Note that there’s no way to split the varieties spoken in these ten villages into two (or however many) languages so that the varieties spoken in one language are unintelligible to the all the varieties of the other language. This is usually called a “dialect continuum”. It is very common:
I must comment. That block of text had so much unnecessary noise.
"You’ve got ten villages in a row. While the changing dialects of neighbouring villages are small and mutually comprehensible, the language of villagers from the first and last in the row have enough accumulative differences to be mutually unintelligible to each other.
You can also have things like languages which diverged from each other very recently, but have become very different in that time, or languages that diverged a long time ago, but only slowly, so they’re still very similar. And, of course, every language has a time continuum, as well as geographic continua: I speak a different language than Chaucer, but the path from Chaucer to me goes through a whole bunch of steps of parents and children who do still understand each other.