OP, I'm very sorry but it's nothing to do with him being your ex, but existing knowledge from your numerous threads.
This person doesn't have anything to do with you. Doesn't want to see, or pay for his own children. Left you 6 years ago. Is not a nice, nor decent person.
You need to work on yourself and move on. It's clear you still love him.
You wanted his motive? Look at these threads. He sends that. And here you are on day 3 of "riddle solving" and "but what does it meeaaannnnnn" on the internet. That's what he wanted. The way you were guaranteed to respond.
Easier said than done, but you need to move on, for your sake.
Incidentally, if my ex, out of the blue, in the day time, had sent me "I love you" I would probably ponder that for an hour or so, wonder what was going on with him, and not respond, as I'm married. Might make me smile, it's nice to hear that someone cares, but that would be the extent of it. The timing and the words would make me think it was genuine. Plus, my ex treated me very well.
Very, very differently, if my ex, at night, had text "love you", I would less smile, more laugh, and quite possibly text back "Hungover? 😄" the following morning.
If I got the latter, like you did, and my ex has treated me like yours has, I would simply think, "not today, satan" and get on with my day.