I think I can understand where Repl.it is coming from. All I know is this behavior (by Riju author) isn't something I would personally want to do. Join a company, leave, and open source something that is directly related to the company's business model, whether it undermines their profits or not. It's standard for companies (other than Apple) to say "work on whatever tech you want, but if it competes with us, we should evaluate it that's OK."
I would at least have gotten their buy-in on the project or idea first. Especially given this commit message:
> repl.it superiority!!
I also wouldn't use intentions or the fact that it currently has less traffic than Repl.it as example of why this is harmless. I would approach this with an empathetic view to how a company would see an engineer leaving and then open sourcing something directly related to the company, based on what they learned while working at the company.