I don't know how much I can trust REW+Umik-1 at these frequencies, but I've measured a maximum of 99dB at MLP from my KF92 at 11Hz. (That's the KC62's 9" big brother with similar DSP). Mind you, that's its peak low-end response, just at the point it rolls off sharply- most I've had at 20Hz is 94dB.
You are in a room, not an infinite open space. The 11Hz waves (so large they can barely be called waves) are being constrained and hence amplified. We're not trying to get 100dB anechoic response here.
And being logarithmic, Xmax drops off sharply with the decibels. Using the calculator again, if Xmax=50mm at 90dB anechoic, then 80dB anechoic needs a more reasonable 15mm.
Given the fancy P-flex surround shenanigans going on, I could believe they can move 15mm. Seen no definite measurements or specs on Xmax though. Not much video evidence. Here you can see someone testing feeding it 12Hz, 20Hz and a sweep, but don't know how hard they pushed the level (link to relevant part):
And the KF92 doing some subsonics (link to relevant part):
Basically, you do get perceptible output down there - you'll sense the infrasonics are happening - but it's not going to shake the building.
Another way of looking at it: Assuming the KC62's Xmax is as good as the KF92's (which doesn't have P-flex), then by driver calculation, the KC62 should be 5-6dB lower than the KF92 (which agree's with KEF's declared 5dB difference) which means it should manage 93-94dB @11Hz in my room.
And the graph at 8:26 of the Small Change video shows 103dB from 2 KF92s at 11Hz. I think that implies a single KC62 would do 90dB in that space (same Xmax, half the drivers and smaller).
But again, how much can you trust any normal measurement mic here? Do they calibrate that low?