This article is a bit of a mixed bag, with some interesting points and some concerning ones.

My chief concern is advising against supplements. My journey into nutrition science started with a B12 deficiency when I'd been eating a varied diet (omnivorous, with plenty of meat and fish) that often (for reasons I don't want to dive into in this comment) included up to ten eggs per day...

In general, regardless of one's diet one should be taking 1000mcg (sublingual) a couple of times a week, or 2000mcg a week, and for most of us this is the cheapest and safest way to ensure sufficient levels. Those megadoses you describe are far out of normal range, and across three continents over the past decade I've had a hard enough time finding supplements with even 1000mcg, so I can't imagine the average person getting themselves into trouble with megadoses unless they're going looking for it!