Got also the message. My account was created in January 2007 and user id is about 700k. So this could be the case.

Iirc, my usernumber is ~ 700k. So, probably an early version of the hash, unless they silently upgrade on logins.

I just got the reset email, and my uid is in the 3.8 million range, created 4/2007.

Could be. My account dates from 2006, and my userid is sub 50k.

Or more likely the database is sharded and they just compromised those physical machines.

That sounds odd, 250k out of 500m sounds like way too little data for even a single shard, no? And why would only a single shard be vulnerable?