if I were to sum up the counter-reaction to a lot of folks here who think that the claimed difficulty is too high:

You're right, under the right conditions. Namely: small site, early in lifecycle, and a perfectly normalized and non-sharded/non-partitioned database.

But I suspect it's more complicated because:

1. they have a ton of traffic

2. they want to maintain a seamlessly perfect UX everywhere

3. their database is denormalized, sharded and/or partitioned

plus possibly:

4. it's gotten at least a little crufty with age (shrug, it happens)