If you post a Youtube link directly into your edit, usually you'll see this, a boring link: https://www.youtube.com/watch?v=U93RImC-by4
To embed, press the Enter key to start a new line, paste the link, then press the Enter key again. This tells the forum software to make an embed, like this:
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Looks like it just generates a picture link to youtube if it's to a short-form video, instead of letting people view it on the forum page.
OK here's a Youtube short as a test:
versus a traditional Youtube vid:
Yeah, just as I thought, the forum software is not "aware" of short-formatted URLs.