I very much doubt Mint is relying on distrowatch as the basis for either of those claims. First distrowatch doesn't try to scale interest relative to Windows or osX..so there's no way you can use those numbers to claim 4th most popular OS. Second, distrowatch stats cannon be scaled to userbase size. So there's no way to claim millions of users from these rankings.
Now the movement of Mint in distrowatch rankiing is indeed very interesting. But distrowatch is not a trendable metric as it stands. The distrowatch numbers are very easily misinterpreted because of the way distrowatch has chosen to represent the average over a long timescale.
For example. Right now Mint is leading on the 6month 3month 1month and 7day averages. This does _NOT_ mean that 6 months ago or 3 months ago or even 1 month ago that Mint was dominant. 3 weeks ago Mint was not leading those long time averages. There is a significant spike in interest ongoing for the last couple of weeks and its big enough to impact even the 6 month average.
Spikes in interest will retroactively impact all the other timescale estimates. People continually misinterpret what those numbers mean. What Distrowatch really needs to do is to take their 7 day average and then provide an historic trending plot of that. There is far more information in the historic trend of the 7day average than the continually updated 6month average.
And even then, distrowatch's 7day average is at best a measure of "interest" or "buzz". Once you are a user of distribution you are less likely to click through a distrowatch page to read about the distro you are already using. People still looking for something different than what they are using are more likely to click on competitors. This makes it very difficult to use distrowatch as a userbase estimate (and they admit as much in their writeup of their stats). It is at best an estimate of "buzz". What is fascinating to me is that distrowatch as a "buzz" metric is completely uncorrelated with google trends metric. The current uptick
in Mint's distrowatch interest level is not mimicked in the Google trends data for "Mint", even though Google trends is itself a different way to measure "buzz."