This is exactly what nearly every other form I've used in the past does. I didn't try this one but it sounds like it redirects back to the form again? If so, I can definitely see double/triple-posting happening.
I know some forums will actually compare contents with the last post you made, and show a warning message "You already submitted this post".
I recall a blog post about TurboTax or some other tax software. The part where it says "checking your returns" -- if it takes longer, people view it as having "done something".
You can probably check a tax return, which is really about 100 numbers, or maybe 10,000, in less than a millisecond. But it's better if takes like 10 seconds.
Someone might know what I'm talking about and have a link ...
I do think it's partially the latency, but the UI concerns you pointed out are probably more important in this case.
* The text doesn't get cleared from the comment box after the comment is sent
* The submit button has no visual affordances to indicate whether it is valid. It looks exactly the same when the form is populated or empty. A conventional UI approach would inactivate the submit button until all the fields are filled.
So despite the message that the comment has been posted, it doesn't appear as if the state has changed as a result of pushing the button.
I think clearing the comment box and inactivating the submit button (which doesn't appear to be a native OS UI element) after submission would help users understand that the state has changed. Alternatively, I think displaying the acknowledgement message on a new page with no submission form would achieve something similar. It could even have a "post another comment" link back to an empty comment page.