We cannot see passwords, so the email is not about the quality of individual passports. Yesterday, we had to upgrade the forum software to a newer version. There was a potential exploit, and the best thing is to get everyone to change passwords. However, asking people is an ineffective way, so the most effective way is to change them en masse. It's not a subtle solution, but it works, though it causes inconvenience.
There probably was nothing wrong with your individual password, but if there's a security risk at a certain point in time, then changing is important- it removes the risk. We've updated the software and everyone has a bright shiny new password, so we're all safer. Some will have been inconvenienced when accessing the forum, but when these things happen, time is of the essence.
We're sorry for the inconvenience and are dealing with the access issues people are having. In your case, you changed the password, so that overwrote the one that you had been sent in the email. If you had seen the email first, that password would have worked (for 24 hours). The system will recognise the latest password saved.
Retired English Teacher