Because they are equal in quality, the second match will be very tight and exciting. Any team can score a first goal in the next match will have high opportunity to win in an aggregated result.
I think this is a stretch in my opinion. There's no way Arsenal and Porto's first team has the same quality. You could argue that Porto managed to keep Arsenal out due to their tactics and all, but having a $60 million player in your squad should theoretically make you the better team on paper.
I missed out on the game but this was a crucial game, though I settled for highlights, Arsenal dominated the game fulltime but got unlucky when it comes to goals scoring.
You could also say they got a bit lucky since Porto failed to score more because they had more threatening chances. If their player quality is a bit better and they have more composure, I believe Arsenal can be 1-0 down earlier. That's just football, if you fail to score or create any chances then it means nothing, even if you have 70% possession. Kind of weird Arsenal suddenly failed to score after they battered teams in the PL.