Their wording ("one of") makes it sound like they had up to perhaps one order of magnitude more (1k-10k), but they do not actually give us a useful number, merely say "it wasn't too much" and "one of an unknown number of total clusters had 1k".
> I think I'd have made a different choice
I think you might misunderstand how these things go and the tradeoffs.
At the beginning of the incident, they find themselves with lots of writes in the west-coast master which aren't in the east-coast one, and some in the east-coast one that aren't in the west-coast one.
Orchestrator cannot promote a working master because they have diverged.
Your choices are:
1. Lose data (large unknown amount) by dumping an hour of west-coast data and going back to east-coast data, can be done in maybe 3 hours total by just deleting west-coast and serving all traffic from east-coast for as you rebuild the west-coast cluster.
2. Lose data (small amount) by rebuilding east-coast from a backup so it can be promoted to, replicating west coast data to it, promoting to it (what they did, 24 hours of time), and then try to manually fix up the small amount of lost data after-the-fact (ongoing)
3. Develop tooling to automate the reconciliation of data while the site is down such that the east-coast side can be merge-promoted without a rebuild, probably takes at least 3 days to build and might break everything, but if it works it probably merge-promotes in under an hour.
4. Keep the site down until east-coast data is manually reconciled, still requires rebuilding east to promote west to it, but then requires manually handling some lost writes... probably about 30-40 hours total downtime.
5. Update the application servers to work fine when the us-west DC is the master, probably about 2 months of development with the site down for the duration.
Which choice would you have made instead? I'm willing to bet they went with either 2 or 4 (and if 4, changed to 2 when it took longer than expected). They probably assumed it would take about 4 hours, and then it simply took much longer than expected because computers are complicated, it turns out.