Some additional nit-picks:
"these models are as closed and unknown as their analog originals"
Analog equipment is very rarely closed and unknown, as anyone who
understands electronics can usually look at the circuitry and work out what
it does. Sometimes a manufacturer will encapsulate circuits in resin
or use custom ICs to prevent copying, but that was pretty rare when most
of the `classic' analog compressors were designed.
"If a compressor had a knee setting, relevant settings were chosen"
What is a relevant setting?
"lower frequencies tended to be less attenuated, while DC offsets tended
to be the most attenuated. This seems to point that all plug-ins tested
use some form of RMS or integration method."
This doesn't follow. Most compressors do indeed use some form
of integration, but that's not the reason for these effects.
The lesser attenuation of lower frequencies points to high pass
filtering in the signal used for envelope tracking, and the greater
attenuation of DC offset is likely due to high pass filtering in the
audio path (analogous to DC blocking capacitors in a hardware compressor).