This is my understanding as well, initially they were basically playing in the caves, in parts of the cave that young Thai children in the region commonly do this in. The mistake they made was doing it too late in the year, their assumption likely was the big rains had not started yet and it was probably a clear day, so they thought they were safe. Then it started raining.
My understanding is that when they were done with their excursion they returned to the entrance chamber and found it submerged. Over the next few hours it sounds like they moved further and further back into the cave as water rose. Likely the ledge they were on is much, much further back in the cave than they normally play in, and involved going through narrow passages and tunnels that casual cavers in the region probably never traverse.
There’s apparently a local custom of signing your name at the “back wall” of the cave. If I had to guess, this back wall is FAR closer to the entrance of the cave than where they are now. Oft-times locals won’t have any clue how deep a cave system really is. That’s why Mammoth Cave in Kentucky has continually “grown” over the years, as they’ve linked it to other cave system sand more fully explored it. For many, many years locals to Mammoth Cave just wouldn’t go past certain points, because the caving was too treacherous or it just got too deep. I believe in the 19th century Mammoth Cave was mapped as having less than 75mi of total tunnels, in the 21st century that number is 400 miles.