With their methodology calculations it is showing much lower supply with around 14 million Bitcoins, and they believe almost 4 million is gone, and It's not like I am surprised with this but I would like to see how exactly did they calculate this but they don't provide any explanation about this, but they probably excluded all coins that was not moved in several years.
But that's definitely not the right way to calculate the total supply. Assuming many old timers would have just stored their wallet just like that.
Many would have stored it as collectibles. Many would have stored it in a secure place where they can access it only in emergency cases.
These coins would not have been moved yet should be counted in the circulating supply.
Them, not mentioning the method to calculate their freefloat tells us why we should not be believing them for the number of circulating supply.
There's no correct method proven to find the actual number of bitcoin circulating supply.