I dunno, it seems a wee bit nitpicky. But maybe "could attack OR be killed by the attacking formation" would cover it? Of course, we can then get the odd situation where a Flayed One and a Genestealer is staring at each other and wondering what happens then.

Probably best to remember that this game was written by GW and not Privateer Press.

Well, yes of course its nitpicky. But the point of an FAQ to make the rules clear.

Poorly worded FAQ's don't really help. If someone has to say "oh that's covered by this other rule in the book" you know its not as clear as it could be. Since the cost of changing it is basically nothing why not?

Especially since it appears to have fallen off the latest revision of the rules and therefore needs putting in again anyway.