Yes as I already said in my original post we may have included that clause in his contract because in this season, unlike the last two, we were both in the Champions League and could potentially face one another.
I'm not sure why you're having so much trouble comprehending what UEFA have said, because their statements are in direct conflict.
Firstly:
"Any agreement between the two clubs that this player wouldn’t play against Celtic should Elfsborg be drawn against them is purely between the clubs"
"Both the UEFA Champions League and the UEFA Disciplinary Regulations contain clear provisions which strictly forbid any club to exert, or attempt to exert, any influence whatsoever over the players that another club may (or may not) field in a match."
They've gone from saying that any agreement that a player would not play is purely between the clubs, to saying that it is strictly forbidden for any club to exert any influence over who another club may or not field.
Secondly:
"UEFA would not have any involvement or consideration of this agreement"
"...any attempt to enforce such a provision would be a clear violation of both the UEFA Champions League and the UEFA Disciplinary Regulations and would therefore be sanctioned accordingly."
They have gone from saying that they would not have any involvement in the agreement to threatening action against anyone who tried to enforce it, hence becoming a part of the whole thing.
The issue has never been that he is not allowed to play against, it's that if he does play then we are to receive a set fee for it. Had this not been a problem for UEFA then they would not have released the statement that they did. Hope this clears a rather simple thing up for you.