RDS caps out at 64 TB unless you use Aurora, so 20 TB is totally manageable without sharding.

This product is for Postgres deployments that are so large they need to be sharded. For these use cases, I think 20TB is about normal.

Yes. But for most workloads it is not much for PostgreSQL. You often will not have to shard at all.

that article seems to suggest 20TB total over the dozen deployments in prod.

Sure, but 20TB in “the only database you need” is mere hours or minutes worth of data for many workflows.