I get why you did.
At the end of the day, most couples will holiday with their kids and step kids no problem, and will blend their families in some way at some point, you've accepted that isn't going to happen in a traditional sense, but then your DP gave you a glimmer of hope saying he'd advocated for you to come along, only for his DC to say absolutely not.
It's a rejection, but not one you even asked for, In your shoes I'd have understood it, but still felt it sting a little, I would have been able to keep it to myself and wished them a lovely holiday from a distance, but then all of a sudden there they are, all happy and ready to go on their holidays like nothing has happened, expecting you to feel hunky dory and play happy families, and in that moment you couldn't pretend it was all okay.
I can absolutely trace my unresolved issues back to being let down frequently by my dad at the last minute throughout my whole childhood, and waiting hours for him to turn up (if he ever did), then if he didn't, having to pretend I was okay so it didn't upset my mum. Needless to say disappointment is the biggest trigger point for me getting upset, even now in my forties I'm a stony face bitch the rest of the time but every now and again if my hopes are raised then dashed I have a hard time controlling the unexpected emotion that comes with that, then the guilt that comes with the inability to deal with it rationally.. no one has the right to tell you how things make you feel.
Your partner was completely tone fucking deaf to have even asked the question in the first place, THEN told you about what happened, and THEN turn up like nothing happened. I'd be inclined to walk away though, not because of this incident, but because the bigger picture is you will never be happy in this situation, you will only ever be on the periphery of his life, not an important part of it.