It's even more impressive when you consider the construction.
Normally you see a metal plate or box on the end of one of those parts.
If you ever take the time to smash one up and peel it apart, you'll most likely see:
The heatsink plate is just that, a tab of metal glued into the package. The lead frame is the first heatsink, having the diodes soldered onto it. Tabs are bent up, so that pins/terminals exit the top of the package. The lead frame is really just floating in the package, a small height above that backing plate.
So despite appearances, it's really just a TO-220FP or whatnot, in a different shape, using diodes. Full-packs are notoriously poor at dissipation (e.g., 30W max for a TO-220 held at room temperature!).
It just goes to show how robust junction diodes are, even at high operating temperature. And they'll use every bit of their 175C ratings, too!
Related story:
Back when I was building a 5kW induction heater from scratch, I had a 35A 800V FWB on the supply (rectifying 240V mains). I had it up around 20-30A DC output, on a not-very-good heatsink. Eventually it failed, drawing fault current through one unlucky diode, once its companion died shorted. Amazingly, despite that ~2000A surge, on top of already being at ~175C maximum temperature (or more), 3 out of 4 diodes still measured well (correct Vf, low enough reverse leakage).
Junction diodes, and similar parts like thyristors (SCRs and TRIACs), are impressively robust. They are the only semiconductors that are robust enough to actually be able to protect with fusing: given that you use the correct type (a so-called semiconductor fuse has such an aggressive fast-blow characteristic, that they can clear faults within milliseconds -- on that time scale, it doesn't matter if it's AC or DC!).
(Semiconductor fuses can be used on IGBTs and the like, but only as a safety feature -- the transistor is past the point of no return, after maybe 0.02 ms, but in the ensuing 0.98 ms, plasma expands, fracturing encapsulation and launching sharp bits of shrapnel. The blast and arc flash is reduced when such fuses are used.)
Tim