So first off, Chris Cocks just stated that most D&D players use DDB. Also, Hasbro has reported that DDB now accounts for most of their D&D sales. So those two facts reinforce each other.
Secondly, 20 million active users is highly unlikely, given that the last number we saw for D&D's entire player base was 50 million people over the entire existence of D&D***, which was misquoted by one source as being 50 million current players and then the misquote was widely cited all over the internet. Your drop in the ocean comment is confusing...if you think 20 million players would be a drop in the ocean of the D&D player base, just how big do you think that player base is, and what is your source?
I would be surprised if the current entire player base is 20 million, let alone those using DDB. There might be 20 million accounts on it, though I would like to see a source on that, but even if so, 20 million accounts is a lot different than 20 million active players. For example, just looking at my D&D club, of the hundreds of students who have made accounts, probably only around a quarter are still active. Quite a few try it and then quit.
***Also, if you look at how that original number was framed by Hasbro, they didn't even use the word "played," instead describing it as 50 million people having "interacted" with D&D over its life, which could mean almost anything. Keeping in mind that the vast majority of the D&D player base is in Canada, the US, and (to a lesser degree) Britain does anyone think that 50 million+ people are currently actively playing D&D? That seems HIGHLY unlikely, and the game would be WAY more profitable if it were true.