The fact that the Sifre Zutta to 5:27 contradicts R. Simeon's view shows merely that the editor drew also upon other midrashim, including, perhaps, that of R. Eliezer ben Jacob and that of R. Ishmael.
The discrepancy between Sifre Zutta 5:27 and R. Simeon's perspective merely indicates that the editor drew inspiration from other midrashic sources, including possibly those of R. Eliezer ben Jacob and R. Ishmael.