I'm thinking maybe because you used to be together and it ended, she feels like it didn't work out anyway, so why waste time and energy and get hurt trying the same thing again hoping for different results?
It could be the way she feels about it, I mean.
In any case, I think it's important for metamours to get along, if there are problems between the two of you it wouldn't work. She'd have to be fine with you and she can't help it if she isn't.
Time will tell if she will feel the same way about everyone he shows interest to or not. If so, then she probably should work on her issues or reconsider poly. If not, though, well it's good that she was honest about knowing it wouldn't work for her if he tried to be with you again, as much as it sucks for you.