It is true that an inference of SIBO is made if cobalamin and/or folate are low. Further testing (a C&S on the poop) is needed to determine which germ(s) are involved in the infection. If the infection persists long enough, there is indeed damage to the intestines from the bacterial toxins, there is an inability to process cobalamin in the gut, and the end result may well be an IBD dog. And that's why I recommend prompt C&S testing at any good local lab, which is relatively inexpensive compared to ending up with a raging intestinal infection and an IBD dog.
Not everyone can afford a biopsy, not every dog (due to age, amount of weight lost already, general health) is a good candidate for surgical biopsy (especially if the owner(s) can't be around to hover over them 24/7 after the surgery), and endoscopy can't necessarily reach the affected parts of the intestine for a greyhound. However, it's hard to get an internist to work with you on IBD without a biopsy. In our case an ultrasound persuaded our vet and the radiologist that the dog proabably had moderate-to-severe IBD, and after a bit of searching we did find an internist to work with us without a biopsy. All I can advise is that one wants to do everything possible to avoid getting to the point of needing one.
The problem with throwing Flagyl at presumed SIBO is that even if the bacterium is susceptible to it, it may not be given long enough and at a high enough dose to cure it. That's what happened to us. When our dog's problem didn't clear up on first Tylosin and then Flagyl, the vet inferred that he didn't have an infection. I had to keep asking, three times, to get the C&S done. And if I hadn't, I don't think our dog would have survived, since he had two vastly overgrown strains of Clostridium perfringens, one of which produces gangrene, for crying out loud!
I hate SIBO the way most of us on this board hate cancer. Fortunately, I guess most SIBO cases are more easily cleared than ours was. But I still think that timely response is the key. I'm going to shut up now and pack for my week-long trip starting tomorrow. Every good wish to you and your dog for a speedy recovery, luvdogs!