Watching the first couple videos, it looks like his claim, of building Quake from scratch, is not at all what he's actually doing in these videos, which is walking through the existing Quake source.

The 0.x videos are a walkthrough of getting the Quake source to compile and run. He goes on to mention that afterwards there'll be a couple of videos on C programming and then the main series will begin after that.

Got it, ok. Thanks.

Thank you. I would love to see him use twitch for higher value & experience to subscribers.

Confused by this. Can you expand on what "higher value" and "experience" the audience would gain from this being on Twitch?

compared to youtube, twitch is a streaming experience with subscribers can comment on chat instantly.

I think, allowing subscribers to ask or comment instantly in what the caster does atm is an amazing opportunity. And not suggesting the streaming twitch over youtube because of same time follow up (as told in another reply), agreed that it would be really hard to simultaneously replicate him.

I would not recommend game dev on twitch, example: handmade hero, awesome project, super great guy, but it is nearly impossible to follow what is going on.

higher value and experience?