Huh? That's incorrect.
First, there is not 1 "B" vitamin. There are several.
Second, B Vits (plural) are water-soluble. They are far less likely to be toxic that non-water-soluble vits like A, E, D and K.
That said, mega-doses of vits are a bad idea borne of the misconception that "if a little is good, more is better". I take a cheap multi daily with food. Not a mega, just about 100% of the RDAs. Maybe it fills in some gaps in my intake, maybe not. Ether way it's fairly harmless and inexpensive.
A possible confounding variable in the reasearch is the nutritional status of the subjects. Maybe they had B vitamin deficiencies that the suppliments were filling. Anyway, one article is not definative. The final step in the scientific process is repeating the finding at independent labs.
How do I know? BS in Nutrition (Penn State), MS in Exercise Physiology (U. of South Carolina)....and my wife has been a dietitian for 15 years.
(...and I stayed at a holiday Inn Express last night)