I think that with it being a longterm relationship at a young age the breakup is always going to hurt a lot.
Plus in their relationship there was also a business component to it. They had that food subscription box business together and I think I remember it launching shortly before the breakup. Additionally, they were one of the "YouTube couples" and content as a couple formed a big part of their livelihood. And the whole Brit Crew thing was still one of the big money makers for them and seemed to be their main social contact. It can be scary to think that all of it is going to change when they broke up. So I am sure that this also played a role in him feeling a bit more lost or sad about the breakup than in a "normal" breakup where it might be easier to cut ties completely without affecting your work life as well and have people ask and talk about it on public platforms.
Didn't they also make Marcus a godfather to Daisy Mae or am I imagining that? I mean, if Niomi's family did that I think it's fair to assume that they believed this was a relationship that was going to last and it left him dumbfounded when it didn't. If his and her family and friends, and the internet, signalled to them that they are the perfect couple, it could have been hard for himto come to terms with the fact that they weren't.
I am sure he feels differently about their breakup by now. At least I would hope so