What they mean is that taking a complex is important, because some of them act as cofactors for the others. Example:
B2 in a cofactor in the enzyme that activates pyridoxine to P5P in the body. But the other B complex vits do not have the specific and critical needs that B12 does.

B12 is not well absorbed orally, and needs passive absorption in the small intestine, to work well. People with intact intrinsic factor may absorb more, but still empty stomach is essential for the B12.

And yes, one can take methylfolate if one thinks they have the MTHFR polymorphism which reduces folate availability.
There is an RX vitamin called Metanx with all 3 active B's in it, but it is expensive.

Methylfolate can be purchased in 800mcg size from Solgar OTC.
5P5 OTC from NOW brands
Methylcobalamin from several ... Jarrow, and Puritians Pride. I
just tested out the new Puritan's on myself and got high levels from testing after 3 months with 5mg daily (empty stomach). My levels were 1999! (the upper limit of the calibration where the test was done).

Most complexes do not have activated B6-- P5P. If they do they list that in the ingredients.

You can take a complex (not the Metanx one however) with food... and most people do because the complexes can cause stomach upset or heartburn.

There are only a few things needing empty stomach care...
B12 oral (also lozenge type --because you swallow the B12 dissolve in saliva)...R-lipoic acid is another.

When one takes a high dose oral B12, very little is absorbed.
1-10%. The rest is lost. There have been studies showing oral is as therapeutic as injections. I believe this is because taking it every day mimics the way we get it from food. A huge bolus every once in a while is only around in the blood for 72 hrs tops and then the rest of the time, that person is back to square one.
Doctors do not often test those patients they put on once a month injections, and this is a mistake. Many end up on these forums feeling terrible inspite of "doctor supervision" .