The fact that there exist lexicographers who are guilty of the same confusion does not make it any less of a confusion.

What would, then? If every English speaker on this planet only normally used “edible” to mean ‘fit to be eaten’, would they be all wrong?

"-ible" is the same as "-able". (The difference has only to do with which conjugation the Latin verb belonged to.)

So what? “Forgivable” normally means ‘easy to forgive’, not ‘which could be forgiven, at least in principle’. So it's not just -ible words to which that applies.