Agreed. And the fact they are blocking mastodon shows it must be impacting Twitter user numbers.
Thus mastodon cross-posting and reposting is a proxy for bannable offenses, and therefore a bannable offense.
My first thought is that people running Mastodon wouldn't qualify as data controllers, but I am no expert in GDPR laws.
Here is a chart of CD sales
https://www.statista.com/chart/amp/12950/cd-sales-in-the-us/
You do not see any sharp drop off because of Napster.