I believe the term is “Mono” whether it’s encoded as a center-channel-only file or the same file in both stereo L and R tracks.
“Dual Mono” is usually used to describe amplifier circuit topology in which each channel of a stereo pair is processed/amplified separately.
You are absolutely right.
Mono/Stereo refers to the number of distinct sources (in caricature, the number of microphones), not to the number of channels. So here, there is only one source, even if 2 channels render it.
So yes, it is simply “mono”.