I don’t work with merges, so maybe I’m way off base, but I thought they meant, they’re working on another branch or fork, then merging the base branch into theirs every so often to get the newest changes, and then that creates multiple merge commits, which they can’t squash at the end…?

I’m not sure, about that last part, but the rest, I’ve definitely seen with contributors that didn’t know to work with rebases (and unfortunately we’re on GitHub, which only half-assedly supports working with rebases by default).