This goes back to what I said in my post. D&D's alignment system is frequently vague and often outright contradictory. There's a reason why alignment threads asking "what alignment s X?" often get several different alignments as responses (sometimes all 9!): it's because each proponent of X being Y alignment can point to somewhere where the rules say that something involved in X qualifies you as Y alignment.
They're not wrong because the line justifying their position isn't in the book. They're wrong because they assume that those lines are consistent across D&D's definition of any given alignment.
In this case, Lawful Good has been defined to allow behavior such as brainwashing people, or even crazy stuff like slaughtering other Lawful Good people if they have ever been of any alignment besides Lawful Good, "so that they can go on to their reward before they can backslide." Or poisoning people as long as you use extra special poisons that cause more pain than regular poisons but only work on Evil people. And there are of course other sections which say that all of those things are definitely not okay if you're Lawful Good.
It's really all over the place. Nobody can be "right" in these sorts of alignment arguments because usually the canon will respond to the same query with "Yes" on one page and "No" on another, which of course violates the Law of Non-Contradiction.