You are correct sir, but until this new firefox screenshot function is available as a JavaScript API in all browsers, it's the closest anyone can get. And there will always be stragglers with older browsers to account for.

It's also worth mentioning that because BugMuncher uses a big farm of Selenium servers, it's pretty rare that it can't take the screenshot in the exact same Browser / OS as the user. And in those scenarios, BugMuncher uses a scoring algorithm to find the closest match available.