Interesting, but I've different info about this car.
Maybe they took some Seat parts as well, but I don't know for sure. The engine is definetely not from Seat.
The engine is indeed Seat! The magazine feature you posted says so (third column).
Also according to Pablo Gimeno Valledor (Los Seat 124 & 1430) the "barchetta Adidas" was built by Léon de Cos using Renault 8 components, including the engine, but after the first racing season (1971) de Cos replaced the French engine with a Seat twin-cam tuned by the Italian specialist Conrero.