> I do wonder how that differs with individuals on a vegan diet who don't eat any animal products.

It's the same for vegans, or perhaps even better, if you see the links below on cow's milk.

Your parent post talks about how you need cow's milk for calcium and meat for iron, but interestingly enough, cow's milk limits iron absorption. If you search for "cow milk iron" you get a large amount of articles stating this.[1]

Note that the example articles I linked talk about infants and toddlers, because in some societies they are most likely to drink milk. I'm Scandinavian, and here people drink milk into adulthood, but I can't quickly find studies on milk and iron for adults.

Also note that the parent's link doesn't mention cow's milk, but instead suggests you should drink it, even though it limits iron absorption, so it all depends on where you look.

There link between the calcium in milk and bone health has also been questioned. We've all learned that you should drink milk for strong bones, which means that Scandinavians shouldn't have any problems with brittle bones of older people, but it's the opposite.[2]

Another interesting factor is that vitamin c (ascorbic acid) helps iron absorption. A vegetarian or vegan will probably eat vegetables high in vitamin c with their iron sources, which helps absorption.

I've read elsewhere that when an omnivore eats heme and non-heme iron, the heme version is preferred by the body because it's easier to absorb. When you go vegetarian or vegan, you have a two-week period or so of lower iron absorption before the gut bacteria changes and improves absorption of non-heme iron, so that there's no longer an issue.

I've also read that vegetarians and vegans often have better iron values that the general omnivore population. As a single test case, my wife is also vegan. When she donates blood they measure her iron level, and it's always perfect. We eat a mix of fresh vegetables, processed soy burgers, things made from pea protein, and so on, anything that's vegan, so it doesn't seem to be a problem.

But the parent talked about "weird supplement focused diets", which may or may not include vegetarians and vegans and could be talking about people who drink a single everything-included meal-replacement drink all the time.

[1] Some examples:

https://www.aafp.org/afp/2002/1001/p1217.html

"In infants, the introduction of cow's milk in the first year of life is the greatest dietary risk factor for the development of iron deficiency and iron deficiency anemia. Cow's milk is low in iron, and its iron is poorly absorbed. In addition, it decreases the absorption of iron from other dietary sources. Therefore, the strict avoidance of cow's milk in the first 12 months of life is essential in preventing iron deficiency anemia."

https://www.ncbi.nlm.nih.gov/pubmed/22043881

"Consumption of cow's milk (CM) by infants and toddlers has adverse effects on their iron stores, a finding that has been well documented in many localities. [...] The high protein intake from CM may also place infants at increased risk of obesity in later childhood. It is thus recommended that unmodified, unfortified CM not be fed to infants and that it be fed to toddlers in modest amounts only."

[2] https://www.webmd.com/osteoporosis/news/20141029/is-milk-you...

"The risk of any bone fracture increased 16 percent in women who drank three or more glasses daily, and the risk of a broken hip increased 60 percent, the findings indicated.

Lots of milk did not appear to either protect against or promote broken bones in men."