Thank you for contacting Telerik Support.
Currently the limitation in the supported Operating System versions is valid only for the Sitefinity Installer - this is the tool which automatically downloads the different Sitefinity components (Sitefinity project manager, Sitefinity Thunder) and installs them on your machine.
In order to install Sitefinity you can actually skip the Installer, and directly download the Sitefinity project manager as outlined in the KB article
you have properly linked to.
In the meantime our team is working on fixing the Installer tool compatibility as we speak, with a rough estimate to complete this task in the upcoming weeks.
I hope the above information helps. Please do not hesitate to get back to us if you have any additional questions, or need further information, I'll be glad to help.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items