"Transactional" emails simply have some distinctive elements, such as the first and last name of the customer, which make them less likely to be filtered out.
So, there is obviously a difference in those metrics between simple notification 'your account have been hacked. change password' and 'hey, we haven't seen you for ages'. As email system notice high user involvement it will never ever block such emails. Actually, i think it will increase IP reputation.
This is - at least from a spam filter's perspective - a huge email broadcast, more akin to a news letter mailing or a spam run.
And semantics aside, a lot of those 43M users will consider the email to be unsolicited (didn't remember they signed up, don't care about computer security, etc). They will happily report such an email as spam, adding to the training set.