Earlier quoted context omitted.
For some use cases, a Beaglebone black can allow for "real" real time while running Linux. It has two microcontrollers (PRU) that share memory with the main ARM cores. So if the real time part can be satisfied by the PRU, it's a neat solution.
Same idea as linux-rt "hard realtime": Reserve a processor (doesn't matter if it's the same chip or architecture or not). The issue with this is precisely the shared memory. If Linux can interfere with the PRU, then there's no assurance. This breaks hard realtime: No assurance that the system will react timely in a certain way to a certain event.
It does work well enough to, for example, drive large LED display boards, emulate old video cards, processing audio, etc. Things that don't work well with traditional soft rt-linux. I'm sure, as you say, that it isn't good for all use cases.