Certainly worth noting, but I don't see why that would make it derivational.

Plural agreement in Turkish is similar: 3PL is optional and only used (at least according to my class) for clarification-- if the subject is clearly plural already, the verb is just in the bare (3SG) form, but if it might be ambiguous, then it is explicitly marked as 3PL. Oddly enough, that suffix (-lar / -ler [vowel harmony]) actually goes in different places than other agreement suffixes to a small degree. For example, it operates a little differently with negation.

It's something to explore in detail, but I think it's still part of inflection.