If the PCV breather box under the intake manifold is plugged, the intake manifold must be removed to replace it, it can't be unplugged as it's got chambers that aren't easily brushed out with solvent. Gunk can collect inside those and solvent won't unclog it. That's then a bit time consuming. The only time those ever plug is if the owner fails to change oil often enough because otherwise it simply doesn't happen. They're not talking about the flame trap as that's a common maintenance item that should be checked, replaced every 30K miles.
$600 seems a bit steep for that in itself so I'd also assume that they're doing more than just the PCV breather box and flame trap related hoses.