You’re right, it is cheaper. But 
Monzo have never aimed to be the cheapest. They’re taking a different approach, by being able to predict with certainty exactly how much an overdraft until you’re next paid will cost you, something that’s impossible with a percentage rate.
Tom firmly believes people are emotional with money, and will budget better with this information, despite it not being economically the best value option. He’s given a good argument and I back him, even though, at the outset, I was very vocal about 50p/day not being anywhere near the pence that a traditional APR% would cost.