The Nexus 5 has a magnetic sensor to detect when the case cover is closed shut. I suspect this is being triggered by a speaker or some other magnetic element of the MacBook.
This has been an issue with the PowerBook G4 models in the past as well. The iBook G4 models also had this problem but only on the lid button.
I wouldn't be confident in the face of a neodymium fridge magnet being rubbed across the mag stripe of a credit card.
What I'm getting at is that the magnets have ruined my credit/debit cards several times, so I wouldn't consider this a "myth" at all.