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 contradiction between Sifre Zutta 5:27 and R. Simeon's viewpoint suggests that the editor also consulted other midrashic texts, such as those attributed to R. Eliezer ben Jacob and R. Ishmael.