He's right you know - it is now an urban legend that just gets passed around without any critical thought or analysis these days.
All that 'you must only use the charger that comes with your device', or 'you should not use a charger that's rated at more Amps than the device' are both totally, and provably false.
Now, largely thanks to the standardisation of nearly everything on the USB standard, you can use practically any charger on any device.
You could use a 100A charger to safely charge a device that needs 0.8A to charge itself - the device will request 0.8A and the charger will only supply what is asked of it.
One thing is that I would avoid charging something that normally charges at 1A on a charger that's only rated for 0.5A (like a USB2 port on a laptop/PC) as this can lead to the charger getting too warm or 'browning out' and supplying dirty current to the device.