Like most of the other replies to this post I think you didn't read the question.

His interest isn't in having a place to write, his interest is in forcing people to read what he writes. Read this...

   "... substack requires to already have a readership, same for a blog in my own domain ..."

That's not true from the viewpoint of "having a place to write".

If it were for personal growth, showing employers, etc. he could blog on his own domain. He wants Medium to find viewers for his posts but doesn't want to give up anything or make the viewers give up anything.