Do you think BRIAN would have said Justin is lucky he took him back? Or would he have looked at you like you had six arms and a belly button in the middle of your forehead if you made that remark?
No. Canon Brian would never accept anyone making that statement, least of all Justin.
And I think that most of the conversations Brian & Justin had from 301 up until the reunion were coded. By the time Justin entered Brian's office that night, they had already negotiated the terms of their reunification and basically just needed to seal it with that incredibly hot 'Lover's Spit' kissing.
Brian really was lost without Justin and stumbled around trying to deal with it until Justin popped up and started stalking him again. That was familiar to Brian and allowed him to reunite with Justin without breaking his rule,"Don't go after anyone." Justin understood that and I think Brian was grateful for that even though they both knew that Brian was as much to blame for their break up as Justin was.