The incident was late July, 2016, but before the date on that blog post.
I don't actually recall when the new system went live for all users, but the old system was live simultaneously for at least a couple months to ease the transition. I wonder if customer support actually lowered the bar during the transition because of a perceived or actual increase in customers locking themselves out.
[1]: https://blog.fastmail.com/2016/07/25/two-step-verification-a...