Real Sociedad was lucky to only meet Inter as the only strong team in the group stage, but if Sociedad were in group F then I am sure Sociedad would not be able to win the group stage or could even be eliminated from the Champions League.
Not all teams that are undefeated in the group stage and become group winners will succeed in qualifying for the quarter-finals if they have strong opponents in the round of 16. PSG is a strong team and will not be easy to beat, so it would be very unfair if you ignore PSG's chances without any consideration. Real Sociedad certainly has a chance to beat PSG, but PSG's defense is not as weak as you mean.
Real Sociedad was into group D with Inter Milan, Salzburg and Benfica this indeed is not hard group but at that time they are not the favorite team to qualify for the next round because at the beginning of competition people were more likely to predict Inter Milan and Benfica as the teams who will be qualified to round of 16 because previously these teams can able to performing well that Inter Milan is the finalist and Benfica can reach to the quarterfinal but surprisingly Sociedad can appears as strong contender into group D and they were qualified as group champion which this is very impressive and Sociedad even only conceded 2 goals from 6 matches
Sociedad indeed gets hard opponent PSG and from the experience and squad conditions indeed PSG much stronger compared to them and most likely Sociedad journey probably will be ended here and for the first leg PSG as a host and i am sure they will pushing to gets the winning results and if possible win with big scores because if PSG can able to do that then their journey will be more easy for second leg however Sociedad achievement are equal to 2003/2004 when at that time they also can successfully to qualified to round of 16 too