Dragonflight is way too specific to be a mobile game. Especially given it was one of the more obvious themes for the next expansion to begin with. The tiers discussed are also using past naming structures for expansions (standard, epic, etc.). A mobile title is also not going to have a "base" tier. That's reserved for a box purchase. Diablo Immortal has 3 paid tiers, but it wouldn't be flagged as "base."
There's also a big difference between pre-registering for a free mobile game, which the new mobile title is far more likely to be, and something explicitly talking about a "pre-purchase."
The mobile title is more likely to be a more generalist Warcraft title along the lines of Raid that allows for more than just dragon-oriented characters.