When the dust clears, you'll have a harmonious web forum -- in which no information is exchanged.
HN's way of balancing this is to require that comments be civil and substantive. Adding the principle of charity is simply a clarification of what that means.
The 'principle of charity' is not primarily about ethics or tone. It's about intellectual substance. It simply says that people should respond to the strongest plausible interpretation of an argument (or comment or phenomenon). What reason could anyone have for insisting on reacting to a weaker interpretation than they need to? I can think of a few, but none apply to a site that wants civil, substantive discourse.
What's the probability they don't know this?
They're corralling the sheeple:
Get typing your sanitised comments. Engagement up.
Weasel Words are kind words.
Correct isn't politically correct.
Motives are always good. There're no bad people in utopia.
There is no marketing. Interviews aren't marketing. HN isn't marketing.