I think it's because the US did sort of promise to block expansion, but then changed its mind and in now in hindsight it doesn't seem like they managed it very well but don't want to look bad.

Ultimately it's the weakest justification to allow Russia to do what it was always going to do anyway. There's not much to really discuss here unless you thought Russia was a credible place in the last 20 years.

That sounds like a prejudgment. You say Russia would have done this regardless of NATO's actions, but you don't know that. And you're downplaying the importance of NATO's actions and it's influence on Russia's actions.

Russia already shared borders with NATO.

The truth of the matter from Ukraine's perspective is that this never would have happened if Ukraine was a NATO member.

NATO hasn't moved aggressively on Russia.

The motivations for Russia and it's neighbours for their opinions on joining NATO are the same. Russia doesn't invade NATO members. That's why Russia's neighbours want to join and why Russia doesn't want them to.