Since the CX retirement contribution is not a US-registered and qualified retirement plan, it is all considered taxable income immediately upon receipt. I'm guessing (but don't take MY word for it) that if you don't have the ability to withdraw it, it's not income yet. Therefore, if you leave CX and don't get it, you've never paid tax on it, so big whoop. However, when you DO take the money, you'll be liable for tax payment on it as a lump sum.
I see your point, and agree, that if you pay tax on all of it now, but have no ability to withdraw that amount on which you've paid tax, and then leave CX and lose that part that's not vested, you'd have paid tax on money you never earned.