>I remember reading the same when the war broke out and was used by Russian as justification to invade.

Small nitpick: has US invaded Syria in your opinion?

Yes, Russia obviously supports Donbas republics, but most of those republics military consists of Ukrainian citizens. In a way it's similar to how US supports Kurds and "moderate opposition". Such situation is really common for civil wars, but I don't think "invasion" is a correct word to use here.