I didn’t check if it could have been achieved only with Material Theme, but in my opinion the sample shows that you are not limited to what Material Theme provides.
So the way I see it is you can choose:
1. Use MaterialTheme if it fits your needs
2. Use MaterialTheme so that you can use the material components, and add custom stuff like JetSnack does with colors.
3. Define a completely diferent Theme if Material Theme doesn’t fit you needs and/or you don’t want to use Material Components.