This has nothing yet to do with any real decentralisation (allowing other instances of Twitter hosting) but more to do with finally freeing up 3rd party access for better and more innovative client apps e.g. no or different algorithms, possibly excluding adverts, newer features, etc.
We're really waiting to see what happens with their Project Bluesky, and whether, and how, they'd be federating with other social networks.
See
Twitter makes big changes for devs as it eyes decentralized future#
technology #
twitter #
API #
3rdpartydevs 
It could also mean better third-party apps
