You certainly can use each individual part of Rails without having to bring in the rest of Rails. I bring in ActiveSupport all the time on non-Rails projects. For ORM I prefer Sequel, curious why solnik didn't mention Sequel, the creator of DataMapper even said that Sequel was everything he wanted DataMapper to be and I can't help but think that played a big role in the decision to not continue developing it.
You can use Sequel in Rails, I personally prefer ActiveRecord as I'm not aware of a Form Helper gem that works with Sequel, though to be honest I never really looked. Way more people use AR than Sequel, so Stack Overflow debugging goes much smoother.
I'm not saying he's wrong for leaving the Ruby / Rails ecosystem. I'm saying his stated reasons are inconsistent with his course of action. I'm fine with his real reasons, I just wish he wouldn't frame it as a slight to Rails.