As I have now read your link (of course I had read tempo mental
), I can say I disagree with their definition, or rather that their definition is only valid within the scope covered in the article - the same as Steve's: subdivisions of the beat.
(As for Tempo Mental, Steve focuses on exploring notations for accidental, punctual and unusual variation of the subdivision of the beat. I'm guessing the topic was more oriented towards repeating patterns)
Now, I might be wrong, but a 12/8 over a 7/8 is a polyrythm to me if the two lines run simultaneously. If I'm wrong, if a rythm guru reads this, I'm really interested in what would be the right word, now.
As for the feeling I get when listening to it and the etymology, it definitely is a polyrythm
It looks to me as if that would just cause a long resolution of the two conflicting rythms. I was taught (by several sources) that polyrythms are just different groupings of countings that all land on the same one. For instance, a peice with nine beats in a measure could be counted as a 3-3-3, 5-4, 2-3-2-2, etc.... Accents being placed on the one, they would all land on the downbeat. Of course those beats could be subdivided.