I don't think it has probably grown much. 2019 is 9 years after the game came out. Plenty of time for enough groups to play and get to the various levels to come up with those numbers.
The only thing that might change it is that it's only DDB players, so it doesn't take all the groups that don't use DDB into account, and there might be some small amount of difference in how the two groups play the game. Sort of like how we here who go online to discuss the game aren't representative of all of D&D.
It's all that we have, though, and it's what people are using to try and kill high level D&D for the millionish people who do play to very high levels. Two million if you want to discuss games over 10th level.
We don't know how many people played to high level in 2019 and as you said all we know is the number of people that played to high levels and used dndbeyond. At the point the numbers came from the tool had been out for a year and a half and my hypothesis is that if you had already gotten to fairly high level when the tool was starting to gain traction there wasn't a lot to be gained to switch things over. After all you had already played for six months or a year tracking your character some other way there's not a lot of reason to switch over. That's why I think if you could look at numbers today it would be far more accurate.
I also agree with you that I and millions of others have enjoyed playing to high levels despite the obstacle of lack of support. I think the answer is to provide more high level monsters, suggestions on how to start characters at a higher level rather than cutting off your nose to spite your face. Creating modules at a high level is going to be difficult because they need to be more tailored to the group's builds and level of magic, not to mention powerful abilities.