However, SQ does not exclude *A Golds in C from the SQ C lounges in SIN. Just *G in Y. If LH ran their lounges the way SQ did, it would be something like this:

LH only allows HONs and F pax on LH Group metal to have sit-down meals in the restaurant in the FCL/FCT, which is turned into a separate, private area, but the rest of the lounge is open to F (partners flying F like UA/TG/SQ),

SEN lounges now are the C lounges, and get *G/SENs in C,

C/FTL lounges now get *G/SENs in Y.

The thing is that LH can't do it this way, because LH's F/C/Y mix on planes is very different than SQ.

A333: SQ:0F/30C/255Y LH: 8F/48C/155Y
A388: SQ:12F/60C/399Y LH: 8F/98C/420Y

LH has a lot more potential business pax (with guaranteed lounge access) on their longhaul planes than SQ does... so their lounge rules have to be different (or they'd have to make much bigger C lounges). And this is why the SEN lounges are designated as *G lounges.