Postgres is a piece of software. Cloud Spanner/Dynamo etc are managed services. It makes no sense to directly compare one with the other.
The entire point is that every cloud provider has a managed postgres offering, and there's no vendor lock-in. Though, technically, Dynamo does have a docker image you could run in other cloud providers if it came down to that, you'd get no support for it.
There's a reason why Google installed and built their own atomic clocks and put them in their datacenters, it is to facilitate global timekeeping for this type of services. Most likely 99.9% of the time this type of database is overkill, and also likely way more expensive than you need.
https://cloud.google.com/spanner/docs/true-time-external-con...
I think just doing some (not necessary millions) ACID transactions over the globe and have consistent DB is strong value proposition even for small users.
There are a couple of databases out there with ddb compatible interfaces, like scylladb
I think they're directly comparable with this context.