In my younger foolish years I actually did this exact scenario, except at a slightly lower speed 85 kph (speed limit was 80 km/hr), but for a distance of nearly 10 km (between two suburbs). In my case I drafted a double trailer gravel truck which was very slow to accelerate making it easy to keep pace. This was only possible due to the massive low pressure wake at the rear of the vehicle.
I was on a road bike with a max gearing of 53-11. When we were at speed I remember spinning like crazy to keep up, typically I would have to do it in pulses where I would bridge up a foot or two to secure a better position in the draft, then take a quick rest. Adrenaline really keeps you motivated in this type of situation. I even remember a few cars driving up beside and staring in amazement or disbelief - I am not sure which one.
The road is flat, so it keeps puzzling me: how could he reach and maintain such a high speed?
It really depends how fast the vehicle accelerates. It needs to accelerate slow enough so that you can keep pace and remain in the draft. At the time I had no trouble hitting 50 kph on my own for short periods so I had the acceleration needed to keep pace until the draft really started to kick in. At higher speeds it is amazing how much the draft really sucks you along, you really don't need to generate crazy power, just have enough leg speed so that you can keep pace in your largest gear.
At 80+ kph you need at minimum 53-11 gearing. Even then I could barely stay on top of the gear, I had to spin incredibly fast. I had a bike computer at the time, but I never checked the cadence, just my speed which was 80-85 kph. At that time (late 1990's) you didn't have many data loggers, which record every moment in detail for posterity like we do today, so I can't offer any more detail than this.
Is it even possible to do this without any external help, e.g. grabbing the truck? Is it the "air-dragging" effect of the truck is what makes it possible?
As long as you keep close to the back of the truck the draft literally sucks you along. Once in the draft you really don't need a ton of power, just the ability to spin fast so you can keep nudging yourself back up into the sweet spot of the draft. I tried to minimize my risk exposure by drafting to the side of the truck, so I had a shot of getting over to the shoulder quickly if the truck suddenly braked. This meant I lost a bit of the draft, but given the insanity of the situation I felt it was a reasonable compromise. Others seem a bit more brave drafting in the dead centre (pun intended).
WARNING: All this is of course incredibly foolish and could quickly descend into disaster. At the time I was young and clueless.