That depends on the American HS. There’s a lot of variation in quality in the US. I’ve always said that I got a terrific liberal arts education in HS.
Evidently, they cover a lot of ground in different subjects in French prepas, so a French Grande Ecole student would have a pretty wide knowledge base, but no way would your typical Oxbridge STEM major know much about literature or history (unless they were interested in that in their spare time). Don't know where you got that idea from. In their HS, they'd be covering subjects at our HS level (but could already choose). Then in the 2 years of "college" before uni, they study 3-4 subjects at our college freshmen intro class level; typically ones related to a future major so are narrowing even further.