Fair, I skimmed past that line.

I would say good luck then, because Blizzard has required the launcher, with an SSO token (single-sign on, good only one time) generated when you click Play, for recent Diablo series games.

You're either going to need a custom Battle.net launcher (which would itself be third-party software) built that fully implements Battle.net login and game launching just to get around this limitation of the actual launcher, or you're going to need other software which can operate with the existing Battle.net launcher. Custom launchers are not impossible; community-built (and usually open source) launchers exist for other games including EVE Online, Lord of the Rings Online, FFXIV and others.

Otherwise, you're looking at Virtual Machines (more third-party software by the way, and sometimes specifically against the rules and/or detected and prevented by games), or multiple PCs.