Yeah, this is due to the difference between a bulb and a CHE.
This is all IIRC, but a RHP/CHE emits heat differently than say, a regular lightbulb. They emit infrared directly downwards from themselves, which hits your enclosure floor before bouncing. Then having your enclosure's walls be opaque or covered allows the IR to be reflected again (light) or absorbed/retained (dark) like a light or dark shirt on a sunny day.
This shows how they work for human housing to heat a home.
So loss from the sides of the bulb is unlikely to happen since IR works differently than visual light.