In another comment, someone says the trick here is to do a resumable upload. The document is covering the sign at the start of the video, so the uploader could

In another comment, someone says the trick here is to do a resumable upload. The document is covering the sign at the start of the video, so the uploader could have uploaded that section (without any text on the sign in-game), then "resumed" the upload with a different video that contains the URL on the sign and starts after he switches to the Minecraft window.