I didn't hear this directly, so I can only comment based on my interpretation here.
This isn't the best idea vs an NMC at a regular cap hit because the team can just ask Marner to waive the NMC if it's clearly not working (if it's not working for us, it definitely will not work for him). A higher cap hit also restricts the team's flexibility in pulling the trigger on a trade even in a m-NTC situation, so Cox's suggestion here is tantamount to paying Marner more for an NMC in everything but name.