Yes, you need Steam to install and update the game, but that is it. For Steam haters, after installing the game, you can:
1. Restart Steam in "offline" mode, and launch the game from the "Play" link in offline Steam
2. Exit Steam altogether (or reboot your computer and choose not to launch Steam after restart) and then launch HOI4 directly from the executable in the Steam/common/Steam Apps/HOI4 folder
3. Copy all of the game files from the Steam/common/Steam Apps/HOI4 folder to a non-Steam folder and launch the executable from that location
I suspect that you could combine 3. with also deleting Steam altogether (so you would only re-install Steam when you needed to update the game files, e.g., when a patch is released, and then manually re-copy the game files to the other location and uninstall Steam again). Seems like overkill/make-work, but I suspect it would work (but haven't tested it myself, as I have no problems with Steam).