As Oliver2002 points out, Priority Baggage is one of the official alliance wide *G benefits regardless of Frequent Flyer-programme (same as lounge access for instance). I guess even SQ has to comply with this one.
*A webpage with *G benefits
A bit sad to see SK move in the direction to more and more isolate itself from *A with many benefits lately being for SK*G only as opposed to *G.
Question: Do they mean they will not tag baggage for any *G or is this to compensate for the extra benefits you get as SK*G and this benefit will only apply to other FFP-members?
I also doesn´t think it has anything to do with Business Class-check in. Economy Class check-in should put a Priority Tag for *G as well.