That would work fine on an officially compatible groupset, but if you accidentally followed this procedure on a mismatched drivetrain, you could end up with a too-short chain which would cause your drivetrain to explode the first time you ride the bike.
Uhm? That's BS. At worst, you would end up with a chain that might be too long. It's based on the chain ring and rear derailleur; branding plays no role except for the length of the derailleur cage (and pulley for those that buy into that crap) and the number of teeth on the front chain ring. My bike on the trainer is a mix of drivetrain, and it works fine if you follow directions.