In any case. Get A Laywer
As it happens, non-competes tend not to be enforceable in most states, although granted it doesn't say what jurisdiction (or even country) this is occurring in.
We are also asking that the existing non-compete, non-disparagement, and NDA they signed still stand
However, my point still stands that non-competes are rarely enforceable.
Most investors won't sign NDAs, let alone non-competes. It's further example of how weird this situation is - on both sides of the table.
You shouldn't compete (even if it's not enforceable)
I would never sign a non-compete because it's not enforcable (I'm in California). If I really wanted to work on a project and the founders said I had to sign the non-compete with no other option I would tell them in writing that I know it is not enforceable and have no interest in adhering to it, and sign it anyway. If the opportunity later came up to compete, I would. Nothing illegal about it.
People go work at rival companies all the time - right now there are loads of good folks leaving Zynga to work for rival companies in exactly the same space - and vice-versa.