This one’s a bit confusing if you look it up.
Lufthansa doesn’t have its own lounge at SEA, but business class passengers usually get access to a partner lounge. At Seattle, that’s typically The Club SEA lounges.
So yeah, your parents should have lounge access, just not a Lufthansa-branded one.
Also worth noting:
It’s not as nice as Lufthansa lounges in Europe
Can get crowded depending on time of day
Still definitely better than waiting at the gate, especially before a long flight to Munich.