With C arrays, why is it the case that a[5] == 5[a]?

This essentially comes down to the commutative property of pointer arithmetic.

This was also used in David Korn‘s (creator of the Korn shell) 1987 IOCCC entry:

https://www.ioccc.org/1987/korn/korn.c

a[5] is the same as *(a+5) and addition is commutative.