The important thing is you need to be aware of your options and of what they are conveying and how to control them.Unless a score is articulated, defaults need to exist. I've checked three different method books (Palmer-Hughes, Magnate method, Pietro Deiro method book). They all mention chords to be played short and not at full quarter note value.
And for the record, my teacher was strictly against slurring every bass note into the chords. As opposed to "never play long notes" his stance was "slurring every single bass note is not correct. They need to be detached".
You want to avoid your accordion sounding like it is drunk and slurring all its phrases while dragging its feet and falling over them. You want to be conscious of all the choices that are there and that you are making.
And yes, slurring things into a mush for something intended as a brisk dance is not going to work well.
I suspect that both "default" and "legato" here are euphemisms. A typical beginner mistake (and here I dare call it a mistake) is to apply bellows pressure through the bass buttons. That requires taking over the force from the bass button to a chord button and in reverse. But legato is an articulation choice, not something that happens because you are being sloppy. The buttons are under control of their individual fingers; it is hand back and palm that control the bellows pressure and movement through hand strap and bass cover. The time when a finger engages and releases the button is a musical choice that has nothing to do with maintaining bellows pressure or a mechanical dependence on what other fingers may be doing.Now that I think about it more, it wasn't so much about playing the bass note longer, but it was about defaulting to a legato.
When you are playing fast, the timing of the key release has a very direct influence on the onset of the next note because the bellow travels with some inertia: leggiero, a short pause before the onset of the next note, leads to an accumulation of pressure accenting the onset of the next note. That makes for a pronounced and reliable onset. A full legato that releases a note only once the next has been started will start a note with half the available air and will make the timing of the onset both hard to perceive and less reliable.
An accordion, unlike a piano, is not a percussive instrument. Notes last while the button/key is being pressed, and the wind requirements of the instrument make the timing of the release a crucial part of the articulation. Your teacher certainly is right in making sure that you don't develop bad habits that prevent you from making sensible performance choices.
Choices have consequences, so it should be you that is making them, not some motorically convenient habit. And your teacher is certainly right in working with you until your articulation is deliberate and not accidental, and you are aware of it. Your audience has a lot more leeway to listen to effects of what you are doing without the distraction of having to do all those movements yourself, so you need to actively train listening to and controlling fine aspects of your performance.
But when you are in control, it is ok to think outside of the box. Most of the time, nothing useful may come from it. But if there were only one correct way to play a score, we wouldn't need all those musicians.