What prompted this speculation was the new box art, in which it said “Xbox One Console Exclusive”. This is versus the old box art that said, “Xbox One Exclusive”. This could be interpreted (and it was) that the former meant that as far as consoles are concerned, Halo 5 will be exclusive to the Xbox One. The latter meant strictly for the Xbox One with no wiggle room for a possibility that it could launch on the PC.
Microsoft did state a couple of years ago that they had no plans to bring Halo 5 on the PC which is why you can imagine the excitement at the possibility that fast forward two years later, they could. There is also the fact that Microsoft has the Xbox Play Anywhere feature that allows some Xbox games to be played on the PC, so it didn’t seem like it was improbable.
In any case we guess the door has pretty much shut on that so if you were looking forward to Halo 5 coming to the PC, think again. That being said, who knows, perhaps the next installment of the Halo franchise could land on the PC, but maybe don’t hold your breath just yet.