Note the "At that point". The author claims that if you use sinatra and install ActiveRecord, and basically everything else that defines Rails, you are just running Rails without running Rails. He is right.
I like to write my own SQL-Queries, so Sinatra is the better choice. But if you'd want to use an ORM, why not directly use Rails?
But if you'd want to use an ORM , why not directly use Rails?
An ORM (the only example listed by you or the author) does not a Rails app make. There are dozens of ORMs of all types across every language; DHH did not invent the Active Record pattern and Rails' implementation of it is not the only viable option, even in Ruby (consider Sequel's ORM for example).
I like to write my own SQL-Queries, so Sinatra is the better choice. But if you'd want to use an ORM, why not directly use Rails?