I think some of the confusion seems to be players are still thinking of Horizons as Legacy, when Horizons is available in both Legacy and Live on PC.
If you have Odyssey you are playing the Live game; Odyssey players will get access to the Python MkII on August 7th, but you can spend ARX to get early access in May.
If you have Horizons but not Odyssey, and are playing the Live game, then you will need to pay ARX* for the ship.
If you are playing Horizons Legacy on PC or Console, you wont be getting it all.
That's my understanding at least.
*which if you buy the ARX will cost you almost the same in real cash as buying Odyssey when it's on sale, which is no doubt one of the reasons for FDEV pricing it this way. Even if you don't want to do any of the on foot, assuming they follow suit with other variants, it would make sense to just buy and play Odyssey instead. This will at least help towards consolidating the playerbase into one game universe.