No. I voted the first post up first, then went to the second link (either via an email link, via the “someone has replied to something” link at top right, or via the list at the bottom of the screen), read that, quoted part of it and replied. The quote is clearly of text in that second thread.
It’s not even clear what “You are not allowed to create a post in reply to another post” actually means.
Apparently this is exactly what happens. If you add a general reply to the topic using the blue “Reply” button at the end everything is fine. You can then even quote as many passages out of other posts as you like.
If you reply directly to the post of another user using the reply button under the post you will get the error messages because Discourse expects you to use the comment function instead.
I had the same error message and when I tried the different options this was the result.
Correct, by doing so a direct reply to the post is startet, attracting the error message. Havn’t tried the email thing but I’m sure @dieterdreist is correct.
Looks like the only thing you can do is to copy text out of a comment and paste it into the reply window. If you want it to look good, you can even add the quote manually … like in the good old days … ), for instance