I suspect that this theorem is indeed due to Weil.
"Foundations of Algebraic Geometry" by Weil was published in 1946, but the 1944 paper "Some Properties of Ideals in Rings of Power Series" by Claude Chevalley (Transactions of the American Mathematical Society, Vol. 55, No. 1 (Jan., 1944), pp. 68-84) attributes to Weil the development of the theory around "ideals in polynomial rings" over a decade earlier in "Arithmetique et geometrie sur les varietes algebriques" in 1935 (see footnote on p. 83).
Reading the AMS review it seems the only other possible originators would have been Siegel, or perhaps Noether or van der Waeden. I don't have a copy of Weil's 1935 work, but you might track it down and (if you can read enough French) check for this particular result.