dielectric

I just had a Rheem Power Vent unit installed. The technician installed the copper pipe from the house directly to the two heat trap nipples. The nipples are silver in color so I assume made of steel. I contacted Rheem to confirm if those nipples acted as dielectric unions and their response...

Hi all, Some years ago I installed a brass valve on a hot water pipe just after the heater. It's developed a lot of corrosion - see pic. Almost certain I was told by the vendor that brass to copper was ok. Anyway, I'll replace the valve. Should I install a dielectric union to prevent this...