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?
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40 Reasons Enemies Rages Against you By D K Olukoya
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Images: Ivan Babydov, Karolina Grabowska, Karolina Grabowska, Karolina Grabowska