Going to my local Target, I can find the PHB, Tasha's Cauldron of Everything, and two diffbox sets. I wouldn't be surprised if 60% of Americans had a D&D player in their family at this point, it's mainstream now.
I can at target as well, but Target doesn't exist in much of Rural America. I can find all the other listed items in much of rural America where Target (or gamestores or book stores) does not exist...
Several issues here. You assume everyone buys books. Not everyone does.
The other odd thing is that you seem to assume somewhat even distribution which is not realistic. Both my wife and I play. My wife is running a game for my family which includes 4 nephews, all but one significant other and my sister (who joined mainly for family time).
Families like mine really skew the numbers. Similar to gun ownership, D&D tends to be clustered among groups of friends and family.
Only in the same way that I would assume the even distribution of a console or a video game. In truth, there are families with multiple consoles and in some cases multiple video games. In addition, digital distribution is probably BIGGER and MORE PREVALENT with video games than with D&D currently.
Which is where you should bring skepticism into the game...instead of accepting whatever is said. Where does this 50 million players number come from? The BEST guess I would have without them giving us information (And for PR, they won't) the best guess I have is that it is based upon sales of D&D books. I have not seen nor heard of any surveys regarding actual player numbers going out or being referenced, no studies on percentages or numbers, or any item similar (and Roll20 numbers or similar items are hardly acceptable reference points in the numbers they are talking about with regard to research on player numbers). The closest then would probably be based upon book sales (which appear to be quite robust).
PR statements like what we saw are NOT necessarily stating what is true, but how they can twist certain numbers to reflect what they want them to reflect. Without seeing the actual research or numbers it is based upon, it could mean anything.
HOWEVER, we DO know other items in entertainment which sell in similar numbers and how exposed they are in the market, how prevalent they are in society, and how easy they are to obtain by the general public with such sales numbers.
The skepticism arises when I see one comparable item (much less several, depending on how you read their numbers, are they simply saying they have 25 million players of D&D now and adding on the 25 estimated players of AD&D of yesteryear, or is it a different number) and see how much exposure it has in society today as well as availability compared to the item that is claiming similar numbers.
AS I said, I'll probably use the 50 million players number myself in other topics to bolster evidence that it is popular, but regarding the actual numbers...the evidence I see in the US and around the world (As I travel) don't seem to support their statements if taken at face value. One would have to see WHAT those numbers are based upon and I haven't seen that they are releasing the numbers or even how they arrived at those numbers.