I'm quite convinced it's coming and I think it has been in Microsoft's interest not to say anything about it in order to push Xbox One X sales; a PC gamer/Halo fan who breaks down and buys an Xbox One X (or S for that matter) out of the concern that Halo will never come to PC (or at least not anytime soon) is a reliable sale for a console that is behind its competition and struggles to have exclusives that really stand out in the crowd.
A while back there was an image that went up on Amazon (I think) of a download card for MCC that said Xbox AND Windows 10. Now, that has been taken down and it was said that it was a mistake. I worked for a number of years in marketing and those kinds of assets are usually created with a purpose long before products hit retail shelves--digital or otherwise. I think based on my experience that it's at least plausible that it was an asset which will actually be circulated, but was accidentally used in that instance. I'll admit I could be completely wrong on this.
However, I do find it peculiar that a Halo edition Xbox One S shipped with MCC and Halo 5 and came with 5 on disc and MCC on download. Phil Spencer did clarify that Halo 5 likely wouldn't come to PC (after all it's far from the most memorable of entries), which makes me think about MCC. Why would they ship one download code and one disc? Why not two discs or two download codes? I suspect that MCC may end up a part of the Play Anywhere program and anyone who used a digital code to own the game would automatically be grouped in with this. Of course if they actually said that, a lot of PC gamers would hold off on buying a new Xbox because it was guaranteed they could play Halo on their machines.
When you consider that Halo 5 did not push the series out of the cultural margins its currently stuck in, I think it's reasonable to assume that Halo 6 might be a more true return to form instead of an experimental outing like 5 was. If this is the case, and of course assuming that 6 will be a part of the Play Anywhere program, I think it would be in their interest to put MCC on PC either at the same time the Xbox One X update comes out or shortly thereafter in order to try and promote Halo 6 as we get closer to that release--a similar tactic to what MCC was sort of used for when it came to Halo 5.
Or Halo will be the only first-party title to stay grounded to the Xbox--which may be less desirable, but significantly more likely.