Yes it should be taken seriously, even if the company claimed it was not to be taken seriously. Whether it's enforceable or not or to what it really applies is

Yes it should be taken seriously, even if the company claimed it was not to be taken seriously. Whether it's enforceable or not or to what it really applies is irrelevant. The important thing is that they may sue you for anything you may create while you work there. The question you should ask yourself is, "do I want to find out." My advice, since your boss's response wasn't something like "Oh, that's just required by our investors, we are going to give you an exception on the thing you want to work on.", don't work on any side thing while there, and walk out as soon as possible.