I'm not sure I fully understand the question but you the only limitation to the unsend feature is the message being 'read'. You can send and unsend as many times as you like.
They're asking if the unsend feature exposes whether the message has been read..... but I can see in the screenshots that there's a 'read' indicator anyway, isn't there? So it was a ignorant question and I wouldn't worry about it.
So you can test whether the recipient read your message by simply sending+unsending the message until you can't unsend anymore? :)