It says at the beginning that the Dom's orders are absolute and when the guy tells Miki to hold on Miki reacts mentioning how he couldn't move. So while the whole explanation at the beginning of each chapter doesn't quite clarify i think it's more in that the sub has more control in terms of a healthy relationship which with this other guy clearly is not.
I took that as him assuming they’re meant to be not that they’re actually fated. And also as a sign of his own mental instability bc clearly he’s unstable if he forced someone to be with him. I think he liked Miki when they were younger and then their second sex results came out and revealed they could match up so in his mind it must be fate.
I thought he was still wearing the collar to keep others away and that this forced partner was just in the past. So is fear keeping him attached to that guy? Bc subs have the real power and he doesn’t have to go along with it. There’s no trust and they clearly don’t have an actual partnership so what’s going on?!! It says if forced to wear one without trust the sub can experience physical and mental instability. But he seems fine except when triggered kinda like PTSD. Hopefully the next chapter clarified this more for me.