to be honest, does there need to be an official statement for dating to happen? It feels very un-mangalike yeah, but I don't think it's that uncommon for pairs to form in real life just through the process of mutual understanding and spending time with each other, without need for special "announcement for the change in status" - I personally know several like that at least
as we all are saying here, in practice they already act pretty much as if they're dating anyways
that being said, this is a manga so I'm sure it's going to be explicitly addressed sooner or later