That page lists this as a feature of their service, implying it isn't available in other backend stores:
Concurrent state locking to prevent corrupting your infrastructure state in a team environment
Re:Pulumi – Modern Infrastructure as Code
by terrorbilly on Tuesday April 14, 2020 @12:51PM
(#22868263)
It's specifically the state-locking feature that's missing from 3rd party backends.
https://www.pulumi.com/docs/intro/concepts/state/