Arthur, it's your second sentence that has the correct interpretation. The STF-2(and other Hsu subs)can certainly operate above 90Hz; it's just the internal low-pass filter(which can be turned off)which has a maximum setting of 90Hz. With it turned off the STF-2 would be capable of frequencies above 100Hz and it would be the receiver crossover at 100Hz which controlled the M22/sub blend.
Many of the finest subs are downward-firing and this is no cause for concern.
No, the M22 doesn't have a "sharp drop" below 90Hz. If you're getting this from looking at the curve taken in an anechoic chamber, this doesn't show what happens in the bass in a typical listening room. For example, the M22s in my listening room have good response to below 60Hz.
