Ah what you say is true... upon examination, this curriculum is for the 4-year ingénieur polytechnicien program, which culminates in a Master's degree (diplôme d'ingénieur).
Standard curriculum is three years of bachelor's, then two years of master's.
In the prépa - engineering school track, it is two years of prépa, then three years of engineering school that gives out a master's in engineering.
Thus the first year of engineering school maps to a (third) last year of undergrad, the second to a first year of a master's and the last to the last year of a master's.