Q:

able to reply to future messages

User unihederon found a bug in chat. 

The word reply is present tense, not future tense. 
I know some will say, "but this only works if you edit your message within 2 minutes of posting it, to reply to a message that has already posted within the 1 min 59 seconds of you posting the original message."  So yes, technically it could be considered a reply then, since it's replying to a message not in the future but in the past.  However, this then messes up the UI design of having older messages appear above/on top of newer messages. 

A:

It's more like linking a message to one posted later, than replying. 
You cannot perform the act of replying to a message that is posted after yours, because to reply to a message you must know the ID (basically a timestamp) of the message you are replying to. 
However, there is nothing saying you can't edit a message later, and include the "reply to" tag. 
See that little pen between the username and the message itself?  It indicates that the message has been edited.  The original message almost certainly did not include the reply-to tag at the beginning, or possibly referred to an earlier message, and thus was not a reply to a message posted later. 
So, almost certainly status-bydesign.