The "only Dragonflight" thing is just bet hedging. They don't want to say "this is how flying pre-pathfinder will work from now on" because people will lose their shit as always. Instead they present it as an expansion feature and then will gauge how people like it and decide to either leave it in Dragonflight or continue to apply it to every expansion.
The way that it's set up allows easy expandability: More base models and customization options, additional Dragonriding "talents" or "skills" or whatever the tree/progression is called, etc.
Even if they decide against carrying it forward, it's likely the parts of it will be cannabalized into other things, similar to how scenarios etc. were reused. I could see them making future dangerous max-level zones like the Maw, but allowing the dragonriding style flight with a specific customizable mount.
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Blizzard pretty much never "straight up lies". The problem is exactly what is happening here. People project their own "promised" and "they said"s onto statements and then become convinced it's absolute and then freak out when it doesn't happen.
What was actually said was that they would like to get the talents in front of people for feedback prior to the actual test release.