See, that’s the thing about rules. They don’t ever allow for non-ideal situations. Simply quoting rubrics doesn’t allow for things that just happen for whatever reason. Nobody said anything about the alternate garb being appropriate, preferred or anything.
You’re making a huge deal out of something that rarely occurs, and when it does, there is an explanation. Our parish does happen to have adult sized server robes. Perhaps this place didn’t.
The bottom line is, we shouldn’t make assumptions about the priest’s judgment.