What caused the low rate of enemies?

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In a video I watched, the guy playing is doing a run of No Mercy on Solo Realism Expert mode. Near the end, where you have to wait on the helicopter, he jumped up onto a certain spot and waited, spamming his medkit. While he waited, there were practically no threats, as very few enemies spawned. What caused the minimal amount of enemies?

The video in question:






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40 Reasons Enemies Rages Against you By D K Olukoya




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