The question is about how that section of material design should be interpreted: http://www.google.co.uk/design/spec/components/buttons.html
If it's basically still fine to have a button with a background which is not elevated but flat. It's not said explicitly in the guidelines that it's a don't but all of the examples show a flat button always as a text button with no background.
When creating a screen with build in keypad (mostly buttons with background fill) it may create to much of visual distraction to elevate them.