False hope.
Disagree... DC is a young and inexperienced team and they been getting level cutarse from everybody this season. Even if that was Milan's reserve team (and it wasn't) they came to play and played hard... they didn't gift anything to DC. Playing well and getting a win against a big squad like Milan should go a long way towards instilling some confidence in that team.
It will give them confidence if they could mentally compartmentalize and treat the game as if they played against a Milan that wanted to win... I sadly don't think that those guys are that naive especially if as you say they've been getting destroyed this season. Those guys know what's up....To think that they suddenly found the quality overnight to beat AC Milan would as preacher put it be a false hope that could likely be dashed by the next cutarse (or string of cutarses they get in the MLS).
I think that there are both mental and physical aspects to "playing hard", and physically I don't doubt that Milan would have respected their opponents (to some reasonable extent), but mentally, chances are they didn't care. Individually and collectively I wouldn't be surprised if their heads were not in it... I also wouldn't be surprised if in the 10 days since their last season game that their training sessions (if they had any) have been anything more than a going through the motions sorta cool down... I don't know the facts, but smart money says that the night(s) before the game were not spent inkeeping with the strictest of pre-match curfew protocols.... Things like movement off the ball, and collective discipline in closing space and marking would fall of significantly in a mentally relaxed atmosphere...and those subtle differences allow a team like DC United to rub shoulders with a team like Milan in a game like this...same reason why T&T would probably stand against the U.S. in a WCQ after the U.S has already qualified even if the U.S. appears to be "playing hard, and not gifting us anything"...Then on an even mental playing field we look like we don't belong on the same field....
If this game boosts DCU to a new level of confident winning play, I'd say they did a good job of compartmentalizing what's real and what's not... I guess time alone will tell.