Plant nutrients are locked up in forms the body can't absorb
Some plant nutrients are absorbed less well, notably iron, zinc and calcium from certain foods. Studies of vegans' nutrient status already include this, because they measure what reached the blood, not what was on the plate. Ordinary cooking, soaking, sprouting and fermenting release much of the rest.
Listing the nutrients in plant foods overstates what you actually get from them. Phytates, oxalates and other compounds bind minerals like iron, zinc and calcium, and some vitamins arrive in forms the body converts poorly. Animal foods deliver many of these nutrients in readily absorbed forms, so a plant diet that looks adequate on paper can fall short in practice.
Partly true, and already accounted for. Bioavailability differs between foods. Non-haem iron from plants is absorbed less efficiently than haem iron from meat. Phytates in grains and pulses bind zinc and iron. Oxalates make the calcium in spinach poorly available. Conversion of beta-carotene to vitamin A varies between people. These are real differences, and nutrition guidance for vegans reflects them. Zinc targets, for instance, are often set somewhat higher for people eating mostly plants.
The difference matters less than it sounds, because it shows up in measurements. Studies comparing vegans and omnivores measure blood levels of iron stores, zinc, vitamin A and the rest. Those figures record what the body actually absorbed. If poor absorption led to deficiency, the status data would show it. For most nutrients, in vegans who supplement B12 and iodine, the rates look broadly similar to omnivores.
Preparation also changes the picture. Soaking, sprouting, fermenting and cooking break down phytates and release bound nutrients. Nixtamalisation of maize is a good example. It is a plant-processing technique, and it shows that plant nutrients can be made available rather than that they can't be reached. Bread, tempeh, miso, dal and tortillas are all traditional answers to the same problem. Simple pairings help too: vitamin C taken with a meal markedly increases iron uptake.
Absorption is also not only a plant issue. Calcium from dairy inhibits iron absorption, and the body adjusts its uptake of iron and zinc to its stores. So bioavailability is a reason to eat a varied diet and prepare food well. It is not evidence that plant foods can't meet the body's needs.