30m it's WotC's own estimate of how many people are playing D&D nowadays (I think late 2021?).
40K is the largest number WotC themselves have reported for playtest responses for 1D&D/2024.
IIRC, they noted the numbers were lower for a later 1D&D/2024 UA (20K, I think), but the 40K is definite.
70% is WotC's own figure, as a percentage of people who answer a UA, who must approve of any change.
So these are being multiplied together. 40k is 0.13% of 30m. 70% of 0.13 is 0.091%. That is the percentage of players WotC is relying on to approve stuff.
The numbers are based directly on WotC's own statements.
That is the best information we have available.
EDIT - Just to add to this, we could just assume WotC is delusional in their estimates and only 10m people are actually playing D&D, but it barely makes a difference. 40k just becomes 0.4%, and the "approval" % is then 0.28%, which whilst obviously larger than 0.091%, is still laughable in the same way.