[TibetanT]

I am not an expert with the 968, but I believe the tensioner does not come off until the arm shown in the picture can be removed.

Basically, the roller is attached to the arm you show in the picture with a bolt in the rear.
In reading the factory manual for the 968, the tension on belt must first be alleviated and belt removed. Then, the arm (tensioner sleeve) comes off along with the roller it is attached to. Then, the tensioner is removed along with the housing.
If the tensioner has already been removed, I believe you may have to apply tension (pry) to arm in order for it to slip off (pulling forward).

One thing I read about installation mentions Loctite, so now I am wondering if previous owner/previous mechanic put Red Loctite on the armature which is why the whole arm will not come off.
Try heating the armature to see if that helps loosen it.

One "dumb" question about this procedure is "did you remove the Guide Rail from the water pump before trying this?" I cannot tell from the picture posted because it appears the water pump is still attached and not removed.

Hope this helps, and good luck with this one. BTW, try and get more pictures posted so we all can help you and learn!