Perhaps. I think these words have somewhat flexible definitions, and that thought process leads me to some rhetorical questions.

Questions like: How many channels can a mod file[1] have? Does that number of channels this file has change depending on whether it is being played back with monophonic or stereophonic (or some other arrangement of) speakers? What happens to the count when one or more individual samples within that mod file contain polyphonic data (chords!), themselves?

It also leads to rhetorical statements, such as: When I play a film that has a soundtrack with 8 (ie 7.1) discrete audio channels on a stereophonic pair of speakers with a sane playback chain, I'm definitely still hearing all 8 of those channels. The same happens if I extend that stereophonic system to greater number of output channels, by adding surround speakers (quadraphonic!) or whatever.

Or maybe I take one earbud out and put it back in its charging case, and through the magic of active electronics, good software, and wireless comms, the output deliberately collapses to being monophonic.

However it happens, I've still got an 8 channel of film soundtrack in my ear(s).

And of course, it also works the other way: If the electronic musical keyboard in front of me is said to support 10-voice polyphony, then that's a hard limit on the number of notes it can produce at once regardless of the number of audio output channels.

Multichannel and polyphonic are pretty bendy terms, I think. Maybe that flexibility was wrong at some point, but they've been flexible in this way for so many decades that it definitely seems right to accept the overlapping uses as being correct.

[1]: https://en.wikipedia.org/wiki/MOD_(file_format)