Effective altruism is based on the core belief that all people count equally. We unequivocally condemn Nick Bostrom’s recklessly flawed and reprehensible words. We reject this unacceptable racist language, and the callous discussion of ideas that can and have harmed Black people. It is fundamentally inconsistent with our mission of building an inclusive and welcoming community.
— The Centre for Effective Altruism
My read is that Bostrom had reason to believe that the email would come out either way, and then he elected to get out in front of the probable blowback.
As evidence, here is Émile Torres indicating that they were planning to write something about the email.
That said, it's not entirely clear whether Bostrom knew the email specifically was going to be written about or knew that someone was poking around in the extropian mailing list and then guessed that the email would come out as a result.
In any case, I think it's unlikely that he posted his apology for the email unprovoked.