I have also been in this situation and it turned out they had some weird separation agreement where they still lived in the same house together for the kids, but they could date other people.
I didn't believe it initially until I heard it from her, and turned out she didnt want it at all, but had proposed it as a "solution" when he said he was wanted to split, rather than actually separating. For many reasons that I won't go into here, he had agreed to this nonsense, but unsurprisingly it was an incredibly toxic situation which meant he did ultimately leave.
However I'd think very carefully whether you really want to be connected to this sort of setup when there is still so much entrenched in their relationship, and how that may impact on your wellbeing and the practicalities. It's still early days, you can walk away now!
However, if you do decide to see how it progresses with him, find out the REAL truth ASAP, I.e. how they both see it , not just how he tells it and ask lots of questions!
EG. if they are still living together "for the kids", do the kids know they are separated? Do they have separate rooms? Do they still do family things together? Will you be able to go and spend time with him at their house? Does the ex have her new partner over to their house? How long do they think this arrangement is viable for? What are they "waiting" for i.e. what needs to change before they live apart?
I know almost everyone on here will say it's the oldest line in the book, but in some instances there IS truth and some men do need support and guidance on HOW to actually leave bad relationships, just as some women do.