Thanks for responding!
I just checked the guideline from Material, here it says
"Text labels need to be distinct from other elements. If the text label isn’t fully capitalized, it should use a different color, style, or layout from other text."
Also I noticed that the link you provided was an theming example, not the general guideline.
I also agree that it is better to use all cap buttons most of the time because it makes it distinguishable, but sometimes the tone is just too strong this way and hard to read.
For example when you have a button for sign in, "Sign In With Email" is much better than "SIGN IN WITH EMAIL".
I think it depends on the product and brand. As long as it is distinguishable, easy to read and consistent, casing doesn't matter that much (some langs does not even have caps like my native lang Chinese 😂)
Thanks again for responding. Great post.