As far as I know, you still can't upload video to Twitch: they are focusing on livestreams. You can record your livestream, and then make it available to view, but well edited videos are rare (you'd have to make and edit the video, then stream it, then edit the VOD for length, etc)

Edit: found https://help.twitch.tv/customer/portal/articles/2577667-vide... which seems to indicate that you can upload video, but it seems to be rare on the platform compared to their bread and butter livestreams.

True but they store past live streams so you can watch old videos.

YouTube is still the king of "Let's Play" and edited-walkthrough formats.

That's true. Although not undisputed. Lots of great let's plays on Twitch. You don't have to watch live content. The video archive has tons of content on Twitch. So you can just go to the archive of your favorite personalities and rewatch their let's plays from months ago.

They also have a whole focused site for it - YouTube Gaming

I'm honestly surprised that's still going.