The talk seems to be mostly about all the limitations and workarounds they've had to deal with, because they choose not to shard. Apparently, they have a policy of adding no new functionality to the database, which presumably means additional separate database services being setup for each new feature. That sounds a lot like accumulating tech debt very rapidly, just because sharding is not on the table, for whatever reason.
Yeah, when they mentioned that they couldn't put any more services on their main DB because of this issue I did a facepalm. They are building out explicit tech debt now because they are not sharding.
Of course they considered it, but the tradeoffs didn't match what they wanted to do - plus they found you could scale to this level without sharding.