Technically the lowest of the high impact actions. Combine one Moderate Impact Action with a part of a second Moderate Impact Action and you get more than a plant-based diet, or just do one of the better High Impact Action.
Sure, if you assume the low end of the range provided for a plant-based diet's impact. You'd need to combine every Moderate Impact Action to reach the high end of the range.
You asked for evidence that a plant-based diet is one of the most effective ways. You found it.
If you already don't own a car, don't travel by plane, and buy green energy, then a vegan diet is the next thing on the list.
The study gives everyone a 2.1T emission budget per year in order to achieve the 2 °C climate target. A single trip by plane can produce more than that, as can buying the wrong kind of energy. A EV car alone produce around 1T per year. A meat-based diet can coexist within that budget, same as the EV car. The average win for switching your utility purchase to a emission free source is 1.6T, while switching to a vegan diet is 0.8T. That illustrate the effectiveness between different kind of strategies to stay within that 2.1T emission budget.
"Fair and impartial" being a phrase does not preclude "impartial" and "fair" from having overlapping meanings. English has plenty of phrases like that. "Vim and vigor" are synonyms. "Intents and purposes", "prim and proper", "born and bred", "leaps and bounds", "pins and needles", "movers and shakers", "hack and slash", etc.
Every dictionary I've checked so far has "fair" as a synonym for "impartial" and a definition that is more or less "treating all parties equally" while none have listed "uninterested in the results" or anything akin to it.
