> This is what weekends are for

What I get from the OP's question is that the company owns anything he creates, independently of it being done on work time.

But if it says "while employed", I don't think working on it on weekends makes a difference. Employment lasts until it ends, not only while on working hours.

The idea behind these clauses is that they apply 24/7/365 while you are salaried.

You are giving the same advice as the commenter you are replying to.