@Kaarme
I dunno, I don't think there's really too much reason to not believe she's the strongest (as of now), when the comment was made by part of the Emperor's guard, IDing her by name instead of just someone gawking at her power. She also had basically an entire chapter+ of power showing-off super powerful abilities, which seems like the kind of thing an author would do to their pet OP Necro character. It kinda makes me think of Mihawk in One Piece, he shows up super early in the series to show off some power and then scoots off forever, but to this day he's still the strongest swordsman in One Piece.
Wrt it being weird that someone could just hire the strongest necromancer in the world and comparing it to trying to hire the president of the top adventurer's guild, keep in mind that nobody actually tried to hire her specifically, they just tried to hire the assassin guild and she picked up the job when everyone else had a case of cold feet, because she had something to gain from it. But also, she's not a regular enlisted member of the assassin guild since she's an "honorary member", which is the kind of title an organization gives you when they actively seek you out because of your achievements.
Plus, she's identified as one of "the Three" instead of as someone from the assassin's guild, meaning that she still has more notability and achievements that the author hasn't revealed yet. It's possible that usually the assassin's guild is beneath her grade usually, but she just decided to pick up this job as a way to gain access to battlefields where the corpses of "heroes" (and probably demons) reside.
It's always possible the series goes on longer than the author expects and he needs to write up a stronger necro, but idk I just don't feel like that's the plan at this point.