Rashi: [When Hashem said that He would kill them,] they did not truly seek [to repent to] Hashem; it was enticement of the mouth and falsity.

Radak #1: "Im" is like Kasher (when), like "v'Im Yihyeh ha'Yovel" (Bamidbar 36:4) and similar verses. When Hashem killed them, the remaining sought him, for they knew that they sinned.

Radak #2: When He said that He would kill them,] they sought Him - "Hinenu v'Alinu El ha'Makom Asher Amar Hashem Ki Chatanu" (Bamidbar 14:40).

Malbim: This is like it says "va'Yis'ablu ha'Am Me'od" (Bamidbar 14:39). Then they sought Him and know that Hashem oversees - "Hinenu v'Alinu El ha'Makom Asher Amar Hashem Ki Chatanu" (ibid., 14:40).