Hey, this bug has been around for a while - i cant back track to what version that i started to find this issue, but its still present in 2.21.
The install script was built with HM NSIS Edit 2.0.3
Install path C:\Steam\steamapps\SourceMods\Prime
Source folder started at Prime\
This is the only folder left in the uninstall, but it makes it hard to 'fully' uninstall a product.