"Our review processes are not fit for purpose. We commit to replacing them with ones which acknowledge our entire ecosystem is built on the goodwill of unpaid volunteers, and we must not squander their time or resources. People like you are our lifeblood and we must not lose your trust."

"We admit we used automated scanning here and tried to pass it off as human review. We got caught. Badly. All our future scans will have to pass our own internal reviews before we make demands of extension authors."

these sorts of things

"Statistically your extensions are one of the most used on Firefox. We will handle all related matters with higher priority and care in the future, and are deeply sorry about this."

Why does it matter if they apologize? Are there brownie points that make a rote ineffectual interaction somehow better if that check box can be checked?

> What could the email have said

If the goal is finding the right magic incantation for apology, then answer to your question is “nothing”. If it’s not, then the answer is “almost anything”.