Depending on the level of compatibility (can you for instance use third party clients to use Facebook's Mastodon instance?) this might be a meaningful push towards Mastodon over Twitter.
Don't really see how it'll be decentralized. Facebook is willingly creating a business venture that they will have to let go? I doubt it. They will want to own everything dollar that flows through their product. They are just using buzz words to get their stocks back up.