How can a dutch oven reach 400°C on an electrical stove?

The 300 Celsius you refer to are the air temperature inside the oven. The energy in your oven is quite sufficient to heat a piece of metal to much over 300 (in fact, judging from the color I have seen on my heating elements, they are probably in the 600-700 C range). But the air around them has quite bad thermal qualities, and doesn't heat up well. It also constantly loses heat to the oven walls and to the outside air. So, you cannot really get above 300, which is actually quite high for a domestic oven, most stop at 250.

Your Dutch oven has excellent thermal capacity and is in direct contact with a heat source. Even though it is not a great heat conductor (in comparison with other metals - it is still much better than air) it is not a problem to heat it to 400 Celsius with a 2 kW heating element. You can actually get higher - watch out for reaching the self-ignition point of cooking oils. This doesn't mean that the air above the pan, or the food in the pan, gets to these temperatures (neither does the food in your oven get to 300 inside).