I'm afraid I don't know much about Canton's frequency response specifications. Some companies (Klipsch & Paradigm, to name a couple) refer to a "typical in room" response that takes advantage of the room gain often seen in lower frequencies. Other companies use a -6dB or even -10dB rating for the FR.
Looking for a comparable speaker (dual 8" drivers), the SVS Ultra Tower gets 28Hz at -3dB.
https://www.svsound.com/products/ultra-tower
So, I agree that the 20Hz is not likely a straight-forward -3dB spec.
The 890.2 is the more recent speaker and from the same series (and thus same or equivalent design of drivers) as my 820.2.
I have spent a while trying to establish the differences between these speakers and do not feel confident of anything. They have too many different speakers and do not provide this information. If you call A4L and ask for this information, they will likely refer you to their buyer for Canton speakers who has a pretty good handle on the models that A4L carries.