2004 Subaru Outback Head Gasket Issue?

I recently bought an 04 outback 145k miles on it
from a private seller with supposedly no issues.
Drove it for two weeks and no problems. Past two
weeks we've had overheating issues and have tried
almost everything. Coolant flush, replaced
thermostat, coolant temp sensor, radiator cap,
radiator, bypass on heater core. Isn't leaking from
water pump and no sound from it either. Passed a
pressure test, no leaks. No smoke, no milky oil.
Took it to Subaru dealer to diagnose it and they
seem to think because of a cylinder 4 misfire it's
the heads and we need to do a head gasket repair
job or just replace the engine completely. Does this
seem right? What could be the issue?