Context:
Smoke on the Water: Chorus Riff

Statement
“[Theory geeks might...consider the Ab5 a tritone sub for the V chord (D) leading back the I (G)]”
 ~ Dave Celentano

Question
I’m assuming he means “[tritone-substitution]”

however:
That confuses me
because

Ab5 = {m2 and m6 (relative to the 1 (G))}
(correct?)

So...

How does that amount to anything tritone (...ic) in nature?