Steam was a big success for QL I believe, and a huge amount of id's catalogue is on it, DOOM 2016 and the upcoming Prey is on it too, so I'd find it rather odd if it wasn't, and I too don't want another launcher, so I'm hoping it will be Steam.
If the mention of anti-cheat measures being put into place via a third party organisation is correct, then i'd be surprised if steam was used as the platform.
There's nothing I'm currently aware of that would stop them using a 3rd party system on steam and still use it to ban steam accounts, i just don't think it could dish out a 'vac ban' only a 'game ban'