So I appreciate your opinion that it is impossible, but as a reviewer I much prefer when the tool tries.
Phabricator does "try", in the sense that it examines the interdiff and attempts (of course, imperfectly, because no implementation can be perfect) to track line movement across hunk mutations.
My claim is that all comments which GitHub places correctly, Phabricator also places correctly. And some comments which GitHub drops, Phabricator places correctly (on the same line a human would select)! However, some comments which GitHub drops, Phabricator places incorrectly (on a line other than the line a human would select).
So the actual implementation you prefer is not one that tries, but one that doesn't try! Phabricator could have approximately GitHub's behavior by just deleting a bunch of code.
That's perfectly fine: many other users also prefer comments be discarded rather than tracked to a possibly-wrong line, too. I strongly believe this isn't a good behavior for code review software, which is why Phabricator doesn't do it -- but Phabricator puts substantially more effort into trying to track and place comments correctly than GitHub does.