Fresh Water Flush- Conflicting Info

[Liquid Diet 2]

I have a 2002 Volvo Penta 8.1 Gi sterndrive, raw water cooled, that is fitted with a fresh water flush on the engine. The VP engine manual that came with the boat says to never run the engine while using the fresh water flush, as does the SELOC manual I purchased. Just had service done and the tech was running the engine while on the flush. Said we run all boats on the flush. I asked the head mechanic, who happens to be a very knowledgeable and well respected mechanic, and he stated that it is OK and that cooling water is supplied to the impeller and he verified this because the water was coming out of the drive, not just the exhaust. Is he correct or is the manual correct? I ran boat yesterday first time after service and no overheat, temp was 175 or hair over.