I have never heard Ray Peat mention garlic, but from the studies he chose to cite in his article "vegetables, etc.", it looks like he is not a fan.
Nutr Cancer 1988;11(4):251-7. Cytotoxicity of extracts of spices to cultured cells. Unnikrishnan MC, Kuttan R Amala Cancer Research Centre, Kerala, India. The cytotoxicity of the extracts from eight different spices used in the Indian diet was determined using Dalton's lymphoma ascites tumor cells and human lymphocytes in vitro and Chinese Hamster Ovary cells and Vero cells in tissue culture. Alcoholic extracts of the spices were found to be more cytotoxic to these cells than their aqueous extracts. Alcoholic extracts of several spices inhibited cell growth at concentrations of 0.2-1 mg/ml in vitro and 0.12-0.3 mg/ml in tissue culture. Ginger, pippali (native to India; also called dried catkins), pepper, and garlic showed the highest activity followed by asafetida, mustard, and horse-gram (native to India). These extracts also inhibited the thymidine uptake into DNA.