I’m still not fully convinced, but I will say that I have had a couple of situations where I had an alternator start to go out a few days after giving a jumpstart. I think it does have something to do with the fact that the alternator is usually never subjected to the current draw from engaging the starter. I think it’s probably only an issue with marginal alternators that were on their way out anyways.
Like I said, I’m still not 100% convinced, but now I’ll only charge someone else’s battery with my engine running or give them a jump with my engine off.