But if Hashem creates a creation…
Torah Temimah: Yerushalmi Sanhedrin — Resh Lakish: three denied the certainty of their prophecy because of lack of open confirmation.
English: Rabbi Shimon ben Lakish said: Three denied [the absolute form of] their prophecy because it was not revealed to them that their words would be confirmed.
Language of condition in prophecy.
Moshe’s “if” is the language of one who waits for the sign to appear, not of one who doubts the mission.