That's such a strange attitude. Yes, reputation matters, but there was a time when gorhill was unknown and had none. As far as I can tell, you're just ignorant of this extension, and assuming it's untrustworthy because of your ignorance of its trustworthiness (and likely some preexisting bias).
> Why do they make you use their extension, instead of simply letting you enter URLs on their website?
Isn't it blindingly obvious? They want to put user-friendly reputation-indicators on the page, without making their users go though a lot of extra hassle. This is a fairly common use case services that provide reputation ratings (e.g. https://chrome.google.com/webstore/detail/fakespot-analyze-f... for another example).
If I had to guess, the main audience for this isn't people with good media literacy skills, it's people who tend to gullibly believe sketchy websites (and who have friends or relatives with good media literacy skills who want to help them).