Think of it this way, you get processing for more than the built-in amps if you want to add an external amp, so more a processing bonus rather than an amp bonus
This is fairly common in avrs, tho. I remember when I got my first similar avr, and hooked up all nine available speaker terminals, but could only use front heights or rear surrounds at a given time depending on sound mode employed (pre-Atmos, so just Dolby PLIIx & z). I did hope all nine would magically work, tho....
If you really want to use the full processing capabilities of the avr, just get an external amp (or a few if you want to take advantage of zones, too)
ps In any case believe they market yours as a 9.1 ch avr, so not misleading in that sense.