Unfortunately Google changes the file extension to *.PNG when you download the files directly from the links above. So you can either change the file extensions back to *.kpp by hand.
The brushes are then installed for example as @fizzyflower described in the previous post.
One way to get the brushes without renaming the file extension is, if you have a Google Drive account and the “Backup and Sync” software from Google is installed on your PC, you move the files after you have called the two links above, from the called pages to your Google Drive account (you create a shortcut to your Google Drive folder by clicking on the folder icon at the top of the called Google Drive pages and selecting this option below (in the then opening menu)) and find the brushes unchanged in your local Google Drive folder on your PC from where you now move them to any folder on your PC. Then install them as described above.
It seemed to depend on the OS, that was my first thought too (and at first I downloaded it in Windows 10 using Firefox and got *.PNG’s), but your experiment shows that there must be another unknown factor, and so I tried it with my iPhone, my Android-Phone and also with my Android-VM via Chrome - in all cases I got *.kpp-files.
I also thought it was because Google wanted to do something good for us and take work off our hands, but there seems to be another hidden reason for this behavior. I wonder if Google will roll some dice beforehand.
And then I tested with Opera, Chrome and Edge on Windows 10 and funnily got *.KPP’s. It seems to be a browser thing! Wow, that was weird.