Yeah, I’m afraid that’s a myth. Let me explain what actually happened, there’s a video version of this 
We decided we wanted our initial cards to be flashy for them to work as conversation starters and also clearly say “work in progress”. Similar to car prototypes.
At that point we didn’t have a proper logo or anything so we validated with our card manufacturer that neon colours were a feasible option for prepaid/debit cards (after seeing some neon gift cards by Debenhams). In all honesty we thought we would only produce a couple hundreds of them and then move into an “official” Monzo card design once our visual identity had been designed.
The surprise however was to find out that people really liked our neon cards, so we decided to stick with them. And here we are 
Not as sexy as the trainers story though 