I (grudgingly) admit you’re right pulykamell – I don’t know where I got my information, but my intel seemed good at the time!
Unisons, fourths, fifths, and octaves are so-called “perfect” intervals, and you’re right, would not be referred to as major or minor…if lowered or raised the resulting interval would be called “diminished” or “augmented”.
(Minor intervals lowered a half step are also “diminished” - as in a diminished seventh - and major intervals raised a half step are also called “augmented”)
There is some method to the madness…