A sense of time like a woman?

The part has 350 watts and needs a voltage of 21V in order to suck properly. To achieve these values, you need a correspondingly powerful battery. Batteries with similar specifications can be found in motorbikes, for example. But a motorbike battery is a bit bigger and heavier than the battery in this Dyson. A stronger battery would therefore mean a bigger device and more weight, which is definitely not the goal for a hoover like this.
If you think about it, there is a very powerful battery in here for its size.
In addition, everyone should be aware that the manufacturers' runtimes are theoretical maximum runtimes or can only be achieved under laboratory conditions and are hardly ever reached in practice. In addition, the 20 minutes refers to the runtime in "normal mode". If you vacuum in power mode, the manufacturer even states that the battery only lasts 6 minutes at best. If you switch back and forth between the modes while vacuuming, then you get plusminus 10min and as Rico Schmid already writes, 10min is enough for 60m2, even with furniture. Most furniture can be moved/lifted out of the way with the free hand. For heavier items that take longer to move, you simply switch the unit off and save electricity.
All in all, this is certainly not a faulty design, but simply a user with utopian expectations.

10 min. is enough for 60m2