The professor is probably well known to forum members from his frequently referenced paper, "The Many Faces Of Cobalamin (Vitamin B12) Deficiency" ncbi.nlm.nih.gov/pmc/articl...
He also writes a blog. The first paragraph of the blog post "Bumblebees With B12" reads:
"It was bumblebees again (“Troubles in Paradise”) in the Netherlands. In this case around Vitamin B12 and specialized clinics. Colleague GP Leenstra does not have a good word for it. He has spoken with internists and clinical chemists, and his opinion - published in de Volkskrant - is clear. B12 deficiency exists, and should be treated properly, but B12 injections are not a panacea for 'everything'. He concludes that specialized clinics prescribe B12 injections to everyone passing through. A fantastic revenue model, he thinks. Costs of 1200 Euro per patient for 'society'. Even Long-Covid patients sometimes get B12 injections. Oh dear. I am going to try in this piece to discuss some of the points that have been raised."
read more at gmed.nl/hommeles-met-b12/
In addition here are two videos covering many important B12 issues including the unreliability of MMA testing.