Earlier quoted context omitted.
That's a very good point actually. Though in this case they say the tip-off came from a law enforcement agency (I assume they mean an American one). It's possible however that agency doesn't realise it's a clandestine operation by another US agency. Or they do, but now the operation is complete (this was three years ago) and they want a way of informing yahoo about the breach without admitting who was behind it in th…
It's not US. It's another state actor. Can't say more.
Did Yahoo Mail even use HTTPS? In that case a FISA warrant would just be an extra level of assurance that they got everything from that person's inbox (plus inboxes of 3 hops of everyone they ever emailed). Otherwise they were just an XKeyscore query, probably filtered by US geodata, away from getting whatever email they wanted (plus unlimited hops).