In the submission rules it details that you can't upload Twitch Vods for your submission. What would you do if you get a great run while streaming? How would you upload it?

you can download your videos at your dashboard (access from twitch or just https://dashboard.twitch.tv/u/{username}/content/video-producer putting your actual username in the link), then upload from there to a valid platform; alternatively, use the dashboard export option to export directly to youtube if that's of your will