Ehh... I definitely see the possible pitfals of relying too much in any single private platform, but trying to blame them for putting their work there is not reasonable. It's good have your own website as a backup, but you can't get good discoverability with just that. To promote that as an alternative to a Twitter or Instagram page is not good marketing advice, no matter how idealistic one might be about self-hosting.
To run a business, the owner needs to get it where people are. Unfortunately most people on the internet are on privately-owned social media.
The Fediverse allows for a much needed middle-ground between self-hosted content and public engagement, but even then it still needs much wider adoption to be as viable as a platform for creators as corporate social media is now.