You might have created two accounts and forgotten about the other one. Note that this is about arduino.cc in general, not the forum specifically. So you might have created an account to use for an Arduino Store purchase, Arduino Cloud, or Arduino Project Hub, then haven't logged into that account for some time even though you actively use the forum with a separate account, then that would explain the email.
Please try logging into the website using the email address to which the email was sent, then let us know whether you discover through that that you actually have a second account you forgot about. Note that, in addition to the classic email+password authentication, arduino.cc also allows you to authenticate using a Google, GitHub, Facebook, or Apple account.
Of course we must take security very seriously and be vigilant, but wild speculation is not productive. Unless you have evidence to back this sort of claim, please refrain from making them.