Not this time, at least: they were in Y.
I guess the international / domestic difference in PB for SGs might be more likely to catch out those that travel more internationally than domestically. It never caught me out, but maybe that's because I do almost all domestic travel, so I'm much more used to the domestic rules than the international ones. For me it was a nice surprise when I was able to use the PB lane on my first international flight as SG.
In any case, I think the FAs should have informed them that they were using the incorrect lane and either kicked them to the back of the normal queue (preferable) or, if they wanted to be really nice, let them through with a warning. Doing nothing just reinforces to them that they can get away with doing it, or, if they really don't know that they're not supposed to do it, then they end up none the wiser. Either way, they'll do it again on their next flight.
mrpooky.