Plant protein is incomplete
'Incomplete' describes proportion, not absence. Nearly all whole plant foods contain all nine essential amino acids, and a varied diet balances them across the day.
Animal foods supply all nine essential amino acids in the proportions the body actually uses, while most plant foods run short in at least one: beans are low in methionine, grains in lysine. The body cannot build or repair tissue beyond the limit set by whichever essential amino acid is scarcest, so a diet built on plants alone risks leaving protein needs unmet unless every meal is carefully paired and planned. Meat, eggs and dairy remove that risk without any thought or nutritional bookkeeping.
The term is misleading. Most plant foods contain every essential amino acid; some are simply lower in one or two: pulses in methionine, grains in lysine. Because the body maintains a circulating pool of amino acids, intake is balanced over a day rather than within a single meal.
The idea that proteins must be deliberately combined at each sitting was popularised by Frances Moore Lappé in Diet for a Small Planet in 1971, and withdrawn by her in later editions as the evidence did not support it. Soya, quinoa and buckwheat happen to be well balanced on their own, but an ordinary mixed diet of pulses, grains and vegetables arrives at the same place with no planning at all.
Using a single word in two different senses within one argument, so that a conclusion appears to follow when it does not.
How it applies here: 'Incomplete' is used as though it meant 'missing an amino acid', when in nutrition it means 'present in a lower proportion'. The alarming conclusion only follows on the first reading, while the evidence only supports the second.
Presenting only the evidence that supports a conclusion while leaving out evidence that tells against it.
How it applies here: Rests on the limiting amino acid within a single food, while what determines adequacy is the diet as a whole.