You must have a better source of information. I'm happy to admit that I am being dumb, but your link makes no mention of that route at all, let alone that it is operated by Scotrail.
All the booking sites that I have tried, including Scotrail's, agree that that service is operated by Northern Rail, which takes two bikes per train on a first com, first serve basis.
I would love to find the Scotrail service with 6 bikes as I have a tour in mind.... could you post it?
Thanks