Unless I am mistaken, SPC refers to the Space(bar) key. And it's not clear in the documentation that behind-the-scenes SPC refers to whatever your leader key is.

We could either map it to the actual Space key or change the placeholder text from SPC to something else (maybe Lead).

I am aware that this only affects users using the default theme. But I am sure many would rather extend the default theme and just change the header and the buttons instead of maintaining a complete new theme.