I believe, though I may be wrong as I haven't registered on any other platform, but, I think yes, when you registered for the new platform, you get the free 30 days that all platforms have for new registration. Although, seems odd that you didn't actually purchase that registration... but I've not fiddled with it on the Playstation to know the workings of that end.

And yeah, free trial is per account, not platform. Using the same account which has already previously purchased registration, you would've been ineligible for the free trial. Would've had to set up a new account to do that. And I vaguely recall hearing it can be a pain to switch accounts after that, but I may be mistaken...