Despite its AA+ credit rating

People do not make fun of the US for having their credit rating downgraded nearly enough. Imagine being the wealthiest country in history with control over the most ubiquitous reserve currency on the planet and the credit rating agencies are like “it’s not that we don’t trust you, but we have standards to maintain.”

Vietnam and Mexico could take advantage of the arbitrage opportunity (sell domestic bonds, buy US bonds), but clearly aren't because they think there's no edge?

That's... Pretty crazy. Doesn't bode well for US inflation.