[callmeal]

> if you just fed someone large amounts of protein and fat they would be lean;

A large amount of protein would make them fat - experiments have shown that about 8% of ingested protein gets converted to glucose in the liver:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3636610/

    The authors calculated that ∼18 g (79%) of the 23 g of ingested protein could be accounted for by deamination; thus those carbon skeletons were available for gluconeogenesis and release of new glucose into the circulation. The remainder, presumably, was used for new protein synthesis.

    The total amount of glucose entering the circulation from all sources was calculated to be 50 g over the 8-h period. However, only 4 g (8%) could be attributed to the ingested protein.

[amanaplanacanal]

I think you misread that experiment. It appears that some quantity of protein was converted to glucose, but that it wasn’t dependent in the quantity of protein consumed.