Discourse won’t let you, but it only knows if you’re posting a video if it has time to generate a preview of the post. Meaning if you paste a link to a video and then continue typing, after about 3 to 4 seconds it will have generated a preview and will know you’re trying to post one of them fancy videos.
The way we can get around this is by not pasting the link to the video into your post until you’re actually ready to submit it, and then immediately submit the post after pasting the link.
The same holds true for edits. In order to edit a post with a video, you have to cut the link into the clipboard and out of the post. Make your changes, save the edit, then go back in and edit it again. Now paste the link and immediately save the edit. This one might be harder because there’s a good chance it’ll be cached, meaning you might have less than a second between pasting the link and saving the edit. In that case, I might try pasting a different video link instead, wait a few seconds for it to generate a new preview, and then quickly switch it back to the link you want and immediately save.