I contacted HK support about driving 4 ohm speakers with my avr 210 and they said do it only temporary and it would decrease the life of the receiver. But in their FAQ on the site it says you can use it for 4 ohm.
As the 210 has 50Wx2 and 40Wx5 @ 8 ohms, the expectation that it will double down for 4 Ohms is on the fringe. So I am not surprised by the H/K tech answer. I did not have this expectation even with my 310 (60Wx2, 50Wx5 @ 8 Ohms). The 510 I now use is expected to do this, however (80Wx2, 70Wx5 @ 8 Ohms)) without breaking a sweat. As the 520 has even more power, it's capabilities are not in question.