Okay like, it's complicated by this one thing:
Did Zhan and Jian even confess/make things official? If my memory doesn't fail me they didn't. While Mo and He kinda confessed and are way more official than ZhanJian are imo.
What we usually see in the chapters is Jianyi acting very buddy buddy with Zhanxi even having this "husband and wife" dynamic but any actually romantic occurrences were more like accidents. And, on their side of things, Mo and Hetian literally take things to bed, live together as a couple and all that.
In summary, yes ZhanJian are built to be the main couple, but a bromance instead of actual romance, while HeMo is still the side but are official(?). You could corroborate this by comparing the cactus Jian gave Zhan yo the time Hetian brought flowers to Mo to his job. Very different vibes. Old Xian just likes to cook his main couple under below boiling water temperature.

lemme get this straight zhengxi and jianyi are technically "officjal" and hetian and momo are still in a situatiobship?
to me it feels zhangxi and jianyi pretty much solid they know their feelings
buf like
Hetian and momo are still at odds what they are to each other