Can someone tell me the TRUE reason for hard purple stems?

[Phyto]

There are multiple reasons you can see solid purple stems, genetics are not one of them.
extreme root bound, severe under watering, untreated bug issues, and lack of calcium in their life.
Purple stems means you are torturing your plants. Stop abusing/stressing your plants.
want zero purple stems? try Nectar for the Gods Herc Harvest combined with their Olympus Up.

The plants in the OP’s video are far from looking “tortured”. I’ve had lots of plants with purple stems, red/purple petioles and no deficiencies. Genetics is a.valid possibility.