I think it is fair. He is disadvantaging Charles and Alex. The condition is because it was under the safety car Charles and Alex can't overtake him because he is driving slow. Their logic of it is okay to slow down only applies in full race conditions where he can defend and try to stop them overtaking him.

This is always a problem in F1. If teams don't know how rules are enforced, how are they supposed to abide?!

I do think it’s a bit odd they used the Unsportsmanlike Conduct rule for it, but it’s also a perfectly valid way to enforce a longstanding precedent. Exploiting the Safety Car rules to impede other drivers has been pretty consistently off limits, so it feels like a bit of a storm in a teacup to kick up a fuss over the wording used for the breach. It’s not like the penalty was unusual, a 5s Time Penalty is basically the weakest possible sporting penalty outside things which are effectively warnings.