IMO this should be illegal. It's your property and they should not be allowed to bar it's sale.
The first part of this can be false. If you're 1/2 way to billions IPO then your options can easily be a year or more of salary to exercise.
About the second part if the company gives you that as a signing bonus you're still on the hook to the IRS for the bonus so now you're 10s of 10000s deep on an illiquid position.
The advice giver is super-early, not the target of said advice.
But I agree that the advice is best suited for super-early people.
It's usually done for early employees who join before multiple rounds of funding occur.