I think this section is always going to be up for debate and I would encourage everyone to follow what works for them TBH. Ironically having a low syn rather than free yoghurt is against the advice on the slimming world website so really I think, as I said, each consultant interprets it differently. The 20kcal to 1 syn rule is not something they admit exists so as there would be no other way to syn free foods then they stick to "can't". Like we all agree kcal are kcal and yes some foods fill you up more per volume but there is no genuine reason to specifically exclude yoghurts, why not pineapple? Or other non speed fruits?
Also how are we expected to syn cooked or mashed or puréed fruit? All of a sudden it miraculously gains a syn value suspiciously similar to that rule yet are free foods that, by that principle shouldn't be syn-able.
If your consultant says not and you are happy with that then cool, if they say you can and you don't want to then that's fine too as I see it. Consider it a "tweak"
Also slimming world themselves say the "other foods" section is for things that are free like potato that you just can't do without for the odd meal but that you should try and restrict for SP.