Except those BDFLs give you escape hatches to do whatever you want. Elm removed it. It's the same as Clojure, if they didn't let you use your own Java libraries. Or if you couldn't write C extensions for Python. Rich Hickey isn't going to chastise you for resorting to a Java library if you have to, because despite Clojure being an opinionated language, at the end of the day he's pragmatic, which is why Java interop is so readily available.

Maybe this extreme limitation is necessary in the Elm's author vision.

It's open source. You can always fork and implement your own vision. In fact, you're encouraged to do so.

Did you read the blogpost? Apparently you're not encouraged to do so.

Based on the core team communication, if you fork the project, you are persona non grata in Elm.

But that's the point. The community close to the core team is toxic, according to this post. So, there are incentives for the rest of the community to fork the project and have it their own way. There is no need to be in good terms with the core team.

Burning bridges is a risky approach if you are heavily invested in Elm.

I mean in normal communities it's OK to fork to experiment/support your use case but still participate in the mainstream community.

An act being allowed legally is distinct from an act being encouraged socially.

Forking is not the solution to every disagreement.