Extremely big doubts about this. How would anyone even know? All the hyperscale datacenters built by FAANG must be 99.9% Linux, and those DCs alone would probably be enough to make 20% impossible to hit for windows.
(Scroll down to servers)
Incidentally this is public facing Internet servers. The % -inside- enterprises us much much higher.
Tools like Active Directory allow you to scale enterprise operations, which is important for companies that have hundreds-of-thousands of desktops etc.
In any event your estimate of 99.9% is unlikely. AWS offers Windows based EC2 instances, and one presumes they're not doing that for 0.01% of the market. Azure will likely have an even higher %.
From the data though, not just guessing, I'd say it's about 20% overall.
I can believe private enterprise usage is higher, but I imagine FAANG and similar companies also have quite a lot non public Linux servers. Not trying to rebut your overall point, but I'm not sure the total non public % is much different.
We know that all the top 25 websites are 100% Linux and represent a lot of servers each.
Also a whole lot of that cited report seems to be misidentified. The top Windows site other than Microsoft properties listed is nih.gov, which looks like Drupal on Linux behind Akamai GHost -- all Linux.
I do accept that Windows is probably about 20-25% of server instances and slowly contracting. But public facing webservers and the infrastructure that directly supports them? Nothing close to this.