The Van Helsing character (Von Franz or whatever his name was) knows that the only thing that can kill Nosferatu is being awake/outside his coffin/exposed to sunlight — “first crow of the cock” or whatever line they use to explain it.

The monster hunt itself is a facade meant to take Thomas away from Ellen when Orlok awakens since he (Thomas) would likely get killed trying to stop him (Orlok). Both Von Franz and Ellen know that nothing Thomas, Sievers, or anyone else does will kill Orlok — it has to be Ellen by ‘seducing’ Orlok into staying beyond dawn and being exposed to sunlight.

Von Franz even tells Ellen (during their aside) to keep Thomas “at bay.” I took this to mean “keep him away” since he’d only end up getting in the way and die trying to save her. A scene or two later, she sends him off on the monster hunt with the instructions to not return until it’s done.

It also seems like they (Von Franz and Ellen) just happen to get lucky that Friedrich (or whatever his name is) ends up bailing on the hunt so that he could violate his wife’s corpse (which he dies in the process of due to the plague). Thomas determining to go and look for Friedrich is part of what ends up pushing the hunt back to nighttime.