"I agree to follow unspecified terms in perpetuity, or return the pay I already earned" doesn't vibe with labor laws.
And if those NDA terms were already in the contract, there would be no need to sign them upon exit.
If the NDA terms were agreed in an employment contract they would no longer be valid upon termination of that contract.
Equity adds a wrinkle here, but I suspect if the effect of canceling equity is to cause a forfeiture of earned wages, then ultimately whatever contract is signed under that threat is void.
They’re profit participation units and probably come with a few gotchas like these.