He’s confused in this video about what polyrythms and polymeters are, he turned things upside down.

Polymeters have the same first beat, it’s like squeezing 4/4 and 7/8 within the same unity of time.
Polyrythms get offset, they will need a common denominator to restart together. In the case of 4/4 and 3/4 they will offset until the 12 steps then restart the cycle.