Not necessarily an indication of inauthenticity in itself, but certainly not a positive.
I’ve seen Bowie albums with legit signatures from band members who did not play on them.
Also, there was a 1969 Velvet Underground handbill sold a couple of years ago — a $500 item even unsigned — with autographs from three band members, including John Cale. Cale had left the band by the time of the gig in question. That one makes me cringe.
I don’t doubt you for a second. I’m not a fan, so I wouldn’t know that kind of detail. Just playing devil’s advocate because it does happen that people sign albums they didn’t perform on.