From one of his most recent Reddit posts, and it does indeed seem to be supported by the evidence I can actually understand:
Although Vitamin B12 generally contributes to energy generation, is anti-inflammatory (which further contributes to energy), and is important in memory formation, under some circumstances (particularly in excess) it can cause fatigue and impaired cognition.
One prominent mechanism by which this occurs is that Vitamin B12 can bind strongly to Nitric Oxide. At high levels, Vitamin B12 can cause a Nitric Oxide deficiency.
If binding occurs at lower levels of Vitamin B12 but high Nitric Oxide levels, one can also get a relative Vitamin B12 deficiency despite adequate Vitamin B12 levels since Vitamin B12 bound to Nitric Oxide becomes inactive.
Nitric Oxide has many functions including: 1) helping improve circulation providing oxygen and nutrients to muscles and other tissues in the body 2) having anti-inflammatory effects (though it can be pro-inflammatory under certain circumstances). 3) is a reverse neurotransmitter (traveling from dendrites to axons rather than vice versa) important for memory formation.
So a depletion of Nitric Oxide by EXCESSIVE Vitamin B12 can cause fatigue by reducing oxygenation and nutrient transport to cells of the body. It also can result in higher inflammation which can cause fatigue. The Nitric Oxide depletion plus possible excess Vitamin B12 inactivation plus increased pro-inflammatory signaling can impair mental function.
Lesson: Sometimes an excess of Vitamin B12 can be just as bad as a deficit.