I thought that's exactly what I said?
right, sorry. you're saying the supporting fleet (C) would know the attacker (A) is at least not a pirate, even if the attacker doesn't have a way to provide full identification without the transponder?
but the same would still be true if it was fleet C defending against hostiles, and A (with disabled transponder) coming to support. A could contact C directly to let them know who they are. and even if C can't know for certain that A is telling the truth, they'd still be able to see that A isn't a pirate (or a member of whatever faction it is that is attacking C).
in both cases it comes down to whether C trusts that A is telling the truth. it makes sense that C would err on the side of caution in a battle they aren't directly involved in anyway (they'd like to support their ally, but they'd rather not risk attacking another ally in the process) but i think the same would be true in the case of C defending against a, likely more powerful, attacker (they don't trust the supporting fleet completely, but they also don't have much of a choice than to take the risk, if they want to win the battle).