I’ll say this. Anything coming from a service manager or service advisor (writer, whatever) should not be considered the final word.
Most service managers/advisors have never been mechanics and generally have a very limited knowledge of auto mechanicals.
This is often where a huge disconnect exists at the dealers. The serv. manager/advisor is the fence between the car owner and technician and what the serv. manager/advisor says may not be what the tech recommends.
It is recommended, especially around the 100k miles range that all tensioners, idlers, and water pump be replaced and this is applicable to all cars; not just Subaru.
Generally if a belt breaks the cylinder heads will not suffer any damage BUT a complete valve job should be done when the heads are removed. Cylinder head valve seats seldom are damaged. Usually the intake valves are the ones that will bend because the intakes are larger in diameter. This causes them to contact the pistons whereas the smaller exhaust valves will clear.
Nonetheless, a complete valve job should be performed while replacing any bent valves and chances are the cylinder heads need to be surfaced. Subaru only allows a max. of .002 of an inch warpage and that’s not much.
Any nicks in the piston tops should be filed or ground smooth because sharp edges can cause hot spots. This in turn can cause severe detonation problems; as in some very loud knocking. Hopefully they will realize this.
Hope some of that helps anyway. 