I assume it means an external Timer? Way to isolate Steam Deck users from submitting runs here, especially those who don't have an HDMI capture setup for it.

You can add a timer before you upload. It's the standard.

Problem is, I would have to add a timer to the video through a video editor.

Do not require a timer on a video unless there is a good reason to do so. A good reason might be a game that uses an auto-splitter to track time without loads.

https://www.speedrun.com/support/learn/moderation-rules