I haven't before seen that extension, though privacytools.io categorically states: "There is no known working solution [to fix the WebRTC Leak in Google Chrome], only a plugin that is easily circumvented. Please use Firefox instead."
Do you have reason to believe that they're mistaken?
No, I haven't seen this note. If they don't post details about how it can be "easily circumvented", it's hard to really reason about it. For what it's worth, their test does not "easily circumvent" this extension.
Seems to work, and it looks to be from a trustworthy source, so that's a plus.
Do you happen to know about a reputable way to disable WebGL?
Do you mean WebRTC? WebGL could disabled in chrome config as we know.