As an admin or a moderator the Reply button is still visible because these users are able to reply to a closed topic. I’ve have 2 mods go back and forth with several posts each before one realized the topic was closed - and they wouldn’t get any replies from other users. 
That’s exactly what happens - no matter if an admin or mod logs in from a PC or mobile. However, I have not checked to see if the reply button still shows for archived topics or not seeing that as of now we don’t have any archived topics. 