It's much more nuanced than that and 20 would help more people to ditch the car.
I've commuted on a non electric hybrid and folder, and on 3 different ebikes:
Half the journey is flat and mostly on segregated paths, for which any bike would be fine. The other half is on congested suburban roads and one hairy stretch of about 1/4 mile of 3 lanes of 40 limit with rush hour traffic, where during that 1/4 mile I have to get to the outside lane to turn right. Then nearer me there's a mile long hill that hits 9% just where there's some busy T junctions for side roads with lots of parked cars and a Tesco where people are always turning in.
On the 3 lane stretch I always put some real effort in to get up to around 20mph as it does feel much safer on that stretch than 15mph. That is obviously above the cutoff. On the folder and the hybrid it was doable. On the e-road bike very much so too. On the eMTB (heavy, knobbly tyres) it was very difficult to get over about 16mph and usually bounced around the edge of the cutoff. That stretch feels more dangerous on the eMTB because the speed differential is higher so it effectively reduces the gap for safe lane changes.
On the uphill section things are different again. The eMTB with its increased torque makes it easy to maintain 15mph even up the steepest bit and that feels far safer passing the parked cars and getting past the Tesco entrance. On the e-road bike I can do 10mph on this section, which isn't too bad but the perceived risk is higher.
On the unpowered bikes I'm doing maybe 6mph, 5 if there's a bad headwind (not unusual as that's the direction of the prevailing winds).
Obviously a bike is less stable at just over walking pace which increases risk, both time passing the parked cars, and the space needed.
