That's great news and I hope nobody is spinning this to mean that Lemmy is already dying.

Fantastic news!

Nor did any spam problem eventuate (yet?)

It would seems so, you can see how engagement is still growing regardless of number of accounts declining, that shows spam accounts didn't actually start spamming yet IMO.

many of these accounts never actually materialised as they never went through email verification

But in that case they shouldn't be counted in the account number, do they?