"...a tritone sub..."

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?

so I got to thinking "HOW could this be anything to do with a tritone?

Then I was like "oh duh: Ab, is the b5 of D... so... is the bass doing a D at that point?

So then i look up bass for Smoke on the water

and dude is playing this

(no D whatsoever)
and it seems accurate to what I'm hearing on the original Deep Purple record

so yeah... still no idea what the deal is here

lol... It seems I'm lacking some requisite knowledge

I started watching a vid about this 

and dude was like

"Tritone substitution is... a Jazz chord substitution theory"

I don't know anything about jazz

and then he said I need some understanding of 

Functioning Dominant

Altered scale

Altered harmony

"...or this probably won't make any sense at all."

I was like
"ah I'll just watch while I eat dinner anyway"

nope. dude is right... I'm completely lost.

At about 2:40 of this lesson I explain tritone substitution.

https://www.guitartricks.com/lesson/16068

The tritone part is that the flat 2nd scale degree is a tritone interval from 5th scale degree.  The substitution part is that the flat2 chord is substituted for the the five chord in the progression.  If you expand those chords to dominant 7th chord you find those chord have common tones and you can simply substitute the root notes.

It's more common to jazz because all of the notes of the dominant 7th chord are voiced.  But it's a similar concept in rock and blues, just using simplified voicings.  Basically bII > I is a substitute for V > I.

I explain it in a blues context here.

https://www.guitartricks.com/lesson/13882

"The tritone part is that the flat 2nd scale degree is a tritone interval from 5th scale degree."

oh.
well that's relatively straight forward
(thanks)

however/incidentally...it's interesting to me:
as you elaborated...
you started citing stuff that I have some intellectual understanding of...
but have yet to internalize it into practically knowledge
so a kind of stacking obscurity occurred the more I read.

thus theoretically I have a vague idea of what Dave was referring;
yet practically I remain in the dark;
and just appreciate the perspective here:
things to get into when I get there.