Math time!
The problem is one of loading and compressive strength of the MFT.
I did some measurements, the OD of the bearing surface the knob is ~24mm (probably generous). With the 20mm hole, this gives just ~138mm^2 of bearing surface area. Finding MDF material properties is hard, but I found a reference to a compressive strength of 10MPa. This would result in a limit of 1380N of clamp load before it embeds.
For a M8 screw, a basic online calculator says this clamp load takes just 2.2Nm of torque (the knob). I found some old military study online, it showed for a 2" knob (~50mm), a mean torque of 148 in-oz of torque by a person. This is about 1Nm.
So in theory, it would work. In practice, people can grunt a knob tighter, and the overlap of the knob and hole is so small, that "falling in" to the hole is very easy to happen.
Cheese's washer looks to be about 40mm OD. This give it all the bearing area it could ever need. The axial load capability goes up almost 7X. At that point, you won't crush it, and it's so big it can't fall in the hole.
The Festool knob is just a dumb design. The base should have been much bigger. Even ~30mm diameter and things probably would have been fine. Someone just designed it way to close.