But that part of the rule that you are focusing on that says " a single d6 " earlier also says "Unaugmented strength" ... what I am trying to point out is the melta bombs strength IS 8+2d6. It is not strength 8 at all.
No, that's not strength, that's penetration. You could look at this as "Str. 8 hit that rolls 2d6 for penetration", which is what the rule implies, and is the way that pretty much everyone plays it. Of course, for RAW buffs, that's not what it actually says. So in terms of pure RAW there are two options:
"In practice, any weapon attacking the monolith will roll for armour penetration using its unaugmented strength and a single D6 no matter what."
1) The meltabomb has no strength, so it can only ever do is do is d6 penetration - useless. This reading assumes that the only penetration you can get is unaugmented str. + d6, and nothing more.
2) The rule doesn't make any mention of stopping things that aren't strength, so anything other than a strength modifier or a extra penetration dice beyond a single D6 is fine - which would mean the base +8 pen modifier still works, even though the attack has a technical str. of 0. In this case, the Meltabomb gets 8+d6 penetration. This reading also allows other modifiers like Tank hunters or similar rules to affect the monolith.
Personally, I agree with part two in terms of RAW. Which ever way you slice it though, everyone plays "8+d6" against the 'lith. It's one of those obvious little things that people get caught up in the minutae of, when it's just not worth arguing.
At any rate, I hope that helped somewhat
