I always interpret those Spencer interviews like "I really don't wanna, but Ross and 343i are pushing hard for it!" and to be fair, they are the developers. There is certainly a valid reason to cut the game in two. Like Teddy said, they kinda did it already anyway by making it f2p...
A separate release however might hurt one or the other. SP on sales if it comes out later, and MP in population if that is the one to be post-poned. That said if there is a separate release for the two modes, than it would be better to focus on the campaign first, because Hi's MP being f2p has a greater chance to recover in the long run.
Doing so could be an advantage as well though! XBox is in desperate need of exclusive AAA content, especially in the first half of 2021! There's a huge drought after The Medium on MS's gaming landscape, a new Halo story would fill that gap. Maybe release the SP together with a beta for mp coming out a few weeks later down the line, so there is at least an incentive for the multiplayer crowd.
Both sides have valid points!