First of all, that moment in Ep 10 only grants + or - RP with Maya, nothing else. The choice of what you say there is not remembered in the game.
Secondly, even if he did add a variable to that moment when Ep 12 is released, it won't do anything for players who've already done Ep 10 because it doesn't retroactively get activated if you chose the more reassuring response way back when you last played Ep 10. The game doesn't know you did it if the variable is added retroactively. You'd have to go back and play Ep 10 again or cheese it by editing the save but only if you're made aware a variable was added. Also, to my knowledge, DPC never has never retroactively added a variable.
So only things with variables matter as the game remembers these and calls back to them later. If there's no variable then it doesn't which means that choice of what to to say to Maya there doesn't matter at all.
You have got to be joking with this.
The emojis in her messages are proof of absolutely nothing when it comes to be any sort of determinative factor about the future of the throuple. She's doing it because she likes him but the game doesn't track or remember this in any potentially meaningful way, it's just a visual aesthetic for the text exchanges.
Well, if DPC had always intended for there be a way to solo relationships then he clearly didn't plan on making them a significant part of the game. It's 11 episodes in and they still haven't started and who knows if anything will even happen with them in Ep 12. I sincerely doubt that anything's going to happen in the episode such that a solo relationship has begun by the time it gets to the NYE party and even if there is some sort of rift, I doubt we're seeing them break up and then openly get with the MC solo in the short space of time this episode likely covers.
So if anything is happening on that front it may still not be unti S4 before there's any real movement on it and it probably won't get much in the way of content with the few episodes it will have.
Alternatively, perhaps it could be that the MC still has a chance to say it to the other girl. After all, they still want to talk more about the things that have been bothering Josy and they said the MC should be included in that, so it's possible that him saying ILY to one of them gets brought up and you have the opportunity to say it to the other girl there and then. If you haven't said it then this would be the chance to say it to both and maybe there could even some RP attached to it, like if you say it to both at the same time you get +5 and if you say it to the girl you haven't said it to then you get +1 for each.
Ultimately, the MC saying it to one girl first doesn't have to be a be-all-end-all choice because it can just be chalked up to the MC taking the opportunity with that girl when it presented itself and feeling the same for the other girl but just not having had the right moment for it yet.