I don't think that number is even close to accurate. SLE almost certainly makes up more than 15% of the market alone. And that's ignoring all of the other enterprise distributions.
I believe that 2015 estimates from the IDC[1] had RHEL at ~60%, SLE at ~20%, Oracle Linux at ~12% and "Other" at ~8%. But I can't access the document at the moment.
Number of installations is not also necessarily a good metric. Red Hat is a very large company with a lot of money and and a lot of employees working on upstream Linux projects and driving their direction.
There are not that many companies doing the same. Which is why they have a lot of influence over the direction of things.
