Edit: The extension now lets you see lemmy comments on any website!
A couple weeks ago I reached out to the dev that makes the Reddit Comments for Youtube extension and asked him to enable support for Lemmy. The developer was already rewriting the extension and decided to implement Lemmy support as well! Here's the GitHub and FireFox links. It also available for Chrome iirc.
The comments appear after you select an instance in the extension's menu on any youtube video that has been posted to any Lemmy instance with a full url without a timecode. Invidious links and Lemmy comments on invidious instances still aren't supported but since its supported by the Reddit comments it should be possible if someone submits a PR.
The developer doesn't use the api, they're using Reddit's free JSON endpoint
What's the difference?
I don’t really understand it since I know almost nothing about programming. But if I understand it correctly (which I probably don’t) the JSON endpoint is just a static set of information you can pull data from? While the API would be able to read and write data and have more precise information pulled. Not sure though because you can comment and vote from the extension so it’s somewhat interactive still