Text labels can be written in sentence case, as long as the button is clearly distinguishable from elements around it.

No, this is not specified. The spec even says that contained buttons can be in sentence case

Sentence case, e.g. ‘Next section’. I use sentence case for more ‘friendly’ or ‘conversational’ platforms.

Note: Yes, it is sentence case, and yes, there should be a full stop if it was true sentence case — but for the love of all things good and designy, please don’t add a full stop.

Choose sentence case or title case over uppercase. (This being said, Material Design does use buttons with uppercase labels.)