Obama made it very clear that the US took its responsibilities to defend fellow NATO allies from the largest to the tiniest very seriously. While he did say Ukraine was not in NATO, he was sending a very clear message that the US was not going to brook any aggression towards any NATO ally, no matter how small or seemingly "unimportant" that country might be.
I think it's also a jab at Ukraine for not joining NATO as well. From what I've read Ukraine is now asking for help against this Russian invasion, and personally I wouldn't be surprised if any NATO assistance was contingent on Ukraine immediately joining NATO.
If a country being invaded had the opportunity to join Nato, then certain countries would eventually be forced into joining by "separatists" or unmarked militaries. The rules must eliminate even the possibility of corruption, because as humans if we can be sure of any one thing, it's that if something can be exploited, some human will exploit it.