Common sense isn't so common on the Internet... So many comments repeating this pleasant fiction told to students in drivers' ed to set a good baseline, seemingly because people want to believe in the just world fallacy.

Legally yes, you want to be damn sure you leave plenty room in front of you, regardless of how people cut in, because if you rear end someone it will most likely be considered your fault, regardless of the circumstances, especially if you don't have a dash cam. This goes doubly if you're driving a heavy vehicle with a longer stopping distance.

But there are plenty of cases where it can certainly be the fault (morally, and legally if you can prove it) of the person who gets rear ended - intentionally "brake checking", insurance fraud "boxing", changing lanes from a stopped lane into a moving lane, or changing lanes with insufficient room behind you and then braking hard (as we seem to have here).