It is simply wrong of that journalist to claim this was a risk specific to Mastodon.
ANY online service can delete your content without warning.
This happens to people on Google who inexplicably lose access to their Gmail or YouTube accounts, for example. And there's usually no appeal or recourse.
Even the biggest social networks in the world that have received billions in funding can lose all their content by accident:
https://www.theguardian.com/technology/2019/mar/18/myspace-loses-all-content-uploaded-before-2016