How is storing data that "Created a registry run key" and "Launched an obfuscated PowerShell script stored in the registry BLOB" in some way "fileless"?

It isn't - unless you want to argue about the semantics of the word "file" where anything (even a script or program) in the registry doesn't count.

This is an odd quote too:
Should you disable PowerShell?
No.
Probably a better answer would be:
Should you disable PowerShell?
Yes. If you don't need it and want to be more secure then do indeed disable it as then PowerShell based exploits will not impact you.