The idea here is that supplements might be "low in quality" (e.g. tainted or ineffective) and therefore you shouldn't be taking them - and since vegans require supplements, that means that you should therefore not be vegan.
Plenty of fortified foods exist, from cereals to coffee creamers, that contain, e.g. B12 (everyone's favorite vitamin as soon as veganism comes up). So the idea that you need to take some low-quality supplement you find on some weird website or something is just not accurate. And if your argument is that there isn't enough selection - then that would mean that the popularity of this must be increased to gain traction. This is an argument for veganism, not against it.
It's universally accepted that pregnant women should take a supplement to boost e.g. their folic acid levels to prevent complications related to neurological development of their children. There isn't a credible doctor in the world that would advise against taking prenatal vitamins on the basis that "they might be low quality". This is just ridiculous.
There are even some carnists that refer to it as "Diet Insurance". Everyone should take something. Just because you eat a wide range of animal products doesn't mean you magically get everything you need.