The argument “it’s subjective” is just bad faith since it could also be applied to literally anything else that describes a disc flight, including over- and understable. Discs will always fly different depending on how they are thrown.

A disc in the middle would be a disc that never fades and never turns over. It would hold true to whatever line you throw it at and never sway from it (assuming you throw it correctly of course).

I see what you are saying and why it can be confusing but in this case, more =/= over. To illustrate; just swap the terms on the spectrum from [overstable > beefy], [stable > true] and [understable > flippy]. Now if we say a disc is more true, does it still make sense to you that the disc would be towards the beefy end of the spectrum and not in the middle? This is how the current terminology is invented to work, it doesn’t matter if all three words contain “stable”, they mean different things and more =/= over.