Bombshells it was mostly a bilateral nonaggression pact between the Svartalves and the Fomor, not as big as formal peace talks.
Sure, but Bombshells does imply that the Fomor are known to be betrayers (at least known to the Fae). So a lot of people might not really want to be there, expecting major trouble.
All the
really "central" supernatural nations
were there -- Winter, Summer, the White Council, the White Court (the only remaining powerful & widespread Vampire Court).
The powers we know are on the Accords but haven't met (the Jade Court, Drakul, another Dragon -- I guess Pyrovax, an Ukrainian shapeshifter guru, etc.) didn't have a reason to be there, or expected trouble and didn't
need to be there enough to take the risk.
Though some might show up fighting on the Fomor's side in BG.