The 2017 Vizio M series was supposedly last years P series at a lower price.
No. Last year's P-series is also this year's P-series at the same price. This year's M-series is last year's P-series with bits chopped off so they can justify not selling the P-series at a lower price.
What's the deal about one HDMI 2.0 port btw? Why is that important?
If you're buying a set to do 4K HDR, I'd think you'd want to be able to receive that content from more than one external source. Especially if this is intended to be in service for the long haul. As I said, though, it's more that having multiple HDMI 2.0 ports should be a standard feature on a set selling for that price at this time and the fact that this set doesn't feature that is an immediate indication that they are re-purposing old parts for this design.