From what I understand, a crowbar diode is a reverse-biased diode put in parallel with the bus, not in series. In normal operation (correct polarity) it doesn't conduct, since it's the "wrong way", and there's no voltage drop since it's not in series with the circuit.

But when the polarity is reversed beyond the diode's voltage drop (that is, beyond -0.6V in your example), the diode is now the "right way" and conducts, limiting the reverse polarity voltage to the diode's voltage drop (in your example, it won't get below -0.6V).

However, that means that all the reverse-polarity current goes through the diode, in effect a short (like putting a physical crowbar across the battery terminals). If the diode holds long enough, the voltage source's short-circuit protection (be it a fuse, a polyfuse, or something else) should cut the current and all is (mostly) fine.