Manchester United also have the advantage playing at home but it is not a guarantee at all, they need to be at their best if they want to win, they are not so good at their home ground but since Real Soceidad are also not performing too well they can win and progress to the next round.
I agree that there is no guarantee for Manchester United even though Old Trafford is the venue for the 2nd leg.
The 1-1 draw in Spain indirectly made it easier for Manchester United with the good statistics that Manchester United obtained at that time.
Home is indeed a different atmosphere compared to playing away, but as has been witnessed directly how Manchester United has performed so far.