There is not professional support, in the sense of commercial support.
First of all: what is a Linux distribution? In short it is a bunch of pieces that someone put together in a way that the result is a working and usable product.
I think that the point is how Fedora Workstation become a final product (as well other spins and any other software). So the mean of the word supported.
As said in the article, the Workstation team build Fedora Workstation with a set of chosen software and configurations. Then (like every software) it pass a series of tests (some of them are automated, others are performed manually, there is a Quality Assurance team). Then it become a general available product. Obviously you can install other software from the repository or any other source, and configure it as you wish.
If you look at the official download site, in the next months, you will find the same ISO you find in the day of the release. You install it and there are a ton of updates. But the community releases an updated ISO more or less each time a new kernel version hit the stable repository (something like Ubuntu point releases). These are called Fedora Respins. But you will not find them in the official download page. Why? Because these ISOs, although working and reliable, weren’t subjected to all the tests performed on the original release. So they can’t be called Fedora releases, formally.
Going back to the article, another point, in my own opinion is: you install Fedora Workstation (that is based on GNOME), then you decide to install another desktop environment. You are free to do that. And it is supposed to work. And you will find help in the community. But, can you still call it Fedora Workstation? If it doesn’t work could you blame Fedora Workstation?
It is like if you install a custom kernel on your Fedora. If it breaks, do you blame Fedora? You will write a blog post that Fedora sucks because YOU installed a custom kernel?
It is (not really) like you buy a “some manufacturer XYZ” car. Then you customize it with spoilers and a different engine. Can you still call it “XYZ”? And if you go to the manufacturer, what they will answer? It is not supported