From my testing, $inpaste will only return $true if the line is sent automatically (i.e. you paste something with Enter attached). Since that doesn't do anything other than move you to the next line in an automatic editbox setup, it won't return $true. If you paste in something in a single line editbox that doesn't include an Enter and you manually hit Enter, it will also return $false. It has to send it without you pressing any key after pasting it for it to be $true.
Now, whether or not that was the intention or if some functionaliry was overlooked, I'm not sure. I think the purpose was just to see if you hit Enter or not, which is useful when pasting something and you don't want your themes to get in the way (using the $inpaste check along with others like $ctrlenter), so it's probably intentional.
