This is true for the most part, unless you adjust keyboard settings in System Settings to let all UI elements be focusable with the keyboard (for tabbing between UI elements). I think it used to be under "keyboard" but they might have moved it in the recent Control Center reshuffle.

Cmd+Space to open spotlight, type in the first 3 or 4 letters of whatever you're trying to do (an application to open, or a system setting to change) and then Return gets me about where I need to go most of the time. Cmd+Tab and Cmd+` for window selection. I don't do much else on the OS itself so my bases are covered.