Earlier quoted context omitted.
From the moon, you don’t need high quality weapons; you can compensate by increasing mass. And that moon base could become independ3nt because of its ability to launch rocks ( https://en.m.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress... )
Independence requires more than the ability to “drop rocks” it requires total self-sufficiency. Without that you’re bombing the people who can kill you just by withholding essentials like food, water, and air. No one is even pretending that a colony on the moon or Mars could be self-sufficient to thst degree with anything like our current technology.
What matters is whether enough money flows in to buy elsewhere what you can’t produce.
If (a big if, but an assumption in Heinlein’s novel) a moon base can be made profitable, and kids get born there, so that people end up thinking of themselves as citizens of them moon, would it be fair if most of those profits keep ending up on earth?
In Heinlein’s novel, the idea is not that those living permanently on the moon bomb those on earth (even though that does happen), but that they use the threat of bombing them to get a fairer deal.