A couple of things,
There is still a risk of assignment, however since NDX is cash-settled, this would only be at expiration.
Since this is cash settled there is no assignment risk as expiration. All cash settled, everything in the money will be exercised/assigned.
It is very unlikely you will be filled at these prices. You are trying to sell a 100 point vertical for 100. If a MM was to take the other side, they would lock in a guaranteed loss. The spread can't be worth more than 100 at expiration, so they will lose commissions, plus they have the potential to lose a lot more.