Nobody, you sound totally clued up even though you must be in tremendous pain.
you are absolutely right that he is behaving like this in shock and dismay at the way you you have reacted. The reason he has floated the possibility of getting back together so quickly is because YOU have behaved in the way you knew you must, from self-preservation and dignity and as advised here.
he can't believe you would be alright letting him go. he was expecting a big fight and tears and begging from you. He was expecting a definite, safe option for him to return to you if he 'wobbled' or found single life (and I'm pretty sure it wasn't single life, unfortunately, love) not what he hoped for.
Often, if there is an OW, she will be panicking now that SHE is lumped with the man and that it is all about to come out, making her look bad and having to deal with the reality of a life with an ex-wife and stepkids and she may be having trouble dealing with his emotional state. This makes things unrosy and DH begins to consider coming home, or at least shoring up that that is an option for him in the future. He wants to return to a situation where two women want him, not none.
I personally think it;s going to be very hard for you to make decisions about this until you know the truth of the situation. But I know you may not be strong enough to cope with that right now. I just think it's going to be hard for you to make your own decisions if things are shadowy.
If there is an OW, the next stage would be that he makes sounds about trying again, and the words are positive, but something about his actions won't add up. He'll sort of try, but look sort of anguished as he does. He'll talk a lot about it being a test or a trial, in a positive way, and that he wants you to do it 'together' and be friends even if it doesn't work out. This is all him trying to make the narrative that he 'tried' and 'it's not his fault' and to pretend like you two are adults making a joint decision so his access to the kids is easier. Then in a couple of months, a 'girlfriend' appears.
The only thing giving you strength and power at the moment is yourself, and the fact that you are not behaving like the lovesick, desperate doormat he might like, so don't give up your power. You are doing incredibly well.
That text sounded horribly ambivalent to be honest. I'm so sorry you're going through this.