I think they only recently decided to implement an official system of deleting older accounts. Perhaps they just started to count the time from that moment on, instead of backtracking every account to see when it was last used? Would save a lot of work
Anyways, I think it's brilliant to finally have something against old inactive accounts. A lot of people create accounts and pets that never get used again. It's a waste of space and more importantly of pet names, coming up with a nice pet name becomes increasingly harder these days.
Logging in once every two years is a really small price to pay to keep your account. Honestly, if you don't access it once in that long period of time, you'll probably never use it anyway. And if you lost your password for it, you won't be able to use that account either, so why insist on keeping it in the first place?