How is it measured? If you use nominal wages, you will see this effect, Malthusian or not, because amount of metal money per capita is inversely proportional to population. You need some sort of GDP estimates to adequately measure wages.

They use baskets of goods. But if the basket weights food heavily, it may see different effects than if it weights manufactured goods heavily. You can call it GDP, but 20th century GDP is definitely measuring the wrong thing. PPP deals with some of these issues, but for each purpose you need a different basket.

Malthusians claim that percent of income spent on food

Do we at least have this data?

This is Engel's law. He had contemporary (19th century) cross-sectional data, not historical. I have heard people claim to have some historical data like this, but I haven't run across it recently. Clark seems to claim to have better knowledge of the basket consumed than PHB, so he ought to be able to graph calories, but I haven't seen him do it. Or rather, he claims to have better knowledge of meat consumed, so he should be able to graph protein, which is another part of Engel's law. Protein consumption changed much more across the 20th century than calories, but I'd be nervous about cross-cultural comparisons.

No, we shouldn't say that the people became 4 times poorer from 2005 to 2008, but we should say that poor people who use wheat as a staple and didn't have (flexible) subsidies did become poorer, while I doubt that's visible in the GDP per capita. Just saying that they became poorer, without quantifying it, is a crude measure, but we're only interested in the sign of the change: did the Black Death make people poorer or richer? It may have had opposite effects on the rich and the poor because they consumed different baskets of goods.