Malarky...that is not a scientific study, that is one vets opinion. Here's a vet that says the exact opposite, and this one is from 2013, not 1982 when the one you posted was written. Feeding Beet Pulp to Horses - Southern States
Too many people use it with fantastic results for it to be as "toxic" as he says. However, as with most things in life, "too much of a good thing can be very bad". You only feed small amounts, they recommend 1 to 2 pounds a day, weighed dry and fed either dry or soaked.
I wish I could show the pictures of how my foster horse is looking after just 30 days of good food and care. I started him out with just a handful of soaked beet pulp and 1/2 a pound of SouthernStates.com: Legends Performance Textured 50lb
4 times a day with 24/7 free choice hay. I worked him up to (and we're holding steady at) 1/2 pound dry BP with 1 1/2 pound of Legend morning and night . He also gets lunch and an afternoon snack of 1/2 pound soaked BP with 1 pound Legends each.
I was having difficulties getting that last little bit of weight up on Faydes spine, so I started giving her a handful in her feed as well...within 2 weeks her spine was noticeably better covered. I highly recommend it, especially if you have an older horse having trouble getting through the winter.