It's just the use of the phrase ratings as it is usually considered a specific output rating by the manufacturer (like it has in the manual a title for the section copied called "Rated Output") and to a specific standard (well, as to how much the FTC standard
still applies check this thread perhaps). Most avrs are not rated for 4 ohm but most are capable of being used with 4 ohm speakers (to an extent). Some test rather well with 4 ohm loads, too but perhaps not for long term/sustained high output levels with every amp channel in use, power supplies in avrs are somewhat limited, particularly compared to large external amps. I have one of Denon's more powerful avrs (4520, here's the
S&V bench test) and wouldn't hesitate to use it for even 4 ohm rated speakers, but it's not rated for 4 ohms by Denon particularly (I currently don't have any 4 ohm rated speakers on any of my avrs). Keep in mind a nominal speaker impedance rating doesn't tell the whole story either particularly.
I wouldn't worry about yours particularly although perhaps if I wanted to setup a full set of 4 ohm rated speakers used as full range without sub all powered by the avr at reference levels in a large room.....maybe then I might worry. Some avrs might have a 2/3/4/6/8 ohm dynamic power rating but that's fairly vague.