That's not quite the reason usually given. Long variable names increase understandability because they can explain the purpose of a variable more explicitly.
temperature_of_the_main_reactor explains more than just
The point is that long tokens decrease readability - they take longer to process, and the increase is super-linear (it takes more than twice as long to read a 20-character identifier than a 10-character one). Therefore there is a trade-off between expressiveness and conciseness, and the sweet spot is somewhat of a subjective value, since people read in rather ideosyncratic ways. This is why there is so much debate about the right way to name variables.
As usual, dealing with a trade-off requires you to follow both principles and decide how much weight to give to either of them.