It says portable, and
> No need to install or uninstall. Just download, decompress and run.
but the main download button is an installer for windows.
That's why you click on "downloads" which takes you to https://github.com/pawelsalawa/sqlitestudio/releases because a homepage button offers people "the most likely installer their OS/Browser combo suggests they probably want", so you click through the full list of downloads to explicitly pick the version you want. Just like you'd do if you wanted to download the Linux and Mac installers even though you're currently on Windows.