Is flat button with a fill background in-line with material design guidelines

1. Why there is three types of button in material design?
2. What are tonal buttons?

Why there is three types of button in material design?

There are text, contained, and toggle buttons. Text buttons are typically used for less important actions. Outlined buttons are used for more emphasis than text buttons due to the stroke. Contained buttons have more emphasis, as they use a color fill and shadow.

What are tonal buttons?

A filled tonal button is an alternative middle ground between filled and outlined buttons. They're useful in contexts where a lower-priority button requires slightly more emphasis than an outline would give, such as "Next" in an onboarding flow. Tonal buttons use the secondary color mapping.