[danaris]

Honestly, the example given in your first quote is a bad one, because a synchronous medium (like a telephone call) is fundamentally different from an asynchronous one (like a chat—or email).

It's actually much more like sending someone an email that just says, "Hi!"

You'd find that weird and off-putting, wouldn't you? Not putting at least some part of the point of the conversation in the initial email completely violates the expectations for how email works.

Chat works fundamentally the same way. Yes, you can treat it as being synchronous—but until you have established an active synchronous conversation (ie, until you and the other person are clearly online at the same time, talking in real time), proper etiquette should be to treat it as asynchronous, like email, and plan one's opening communications accordingly.