yeah but when the last data burst goes out during touchdown it clearly doesn't transmit the last air speed... Most probably the cockpit was the last part of the aircraft that stayed intact and thus could still send data while the aircraft had "landed" already!
As previously pointed out, ADS-B doesn't transmit airspeed at all - only groundspeed (and the GS values are near-real-time, not averaged)
It's also highly unlikely that the speed readouts were from the aircraft once it was on the ground. Mode S uses 1090MHz, which makes it even more line-of-sight than VHF - unless the enthusiast with the Mode S receiver was very close, they would only have received transmissions while the aircraft was airborne and above their radio horizon (as indeed the height readouts indicate).