Use code internethistorian at https://incogni.com/internethistorian to get an exclusive 60% off an annual Incogni plan.----Episode 2: Exploring the world of ...
But to actually answer your question, it takes time to prove (or even notice) when a work has been plagiarized, particularly when the person who did wrong does not mention, or intentionally hides the original source. The Hbomberguy video is about exploring that in depth and the IH video is just one example, not the main topic.
Here is a thread from 7 months ago where more people noticed the video was plagiarized due to a DMCA of a re-upload: https://www.reddit.com/r/youtubedrama/comments/1391d4o/internet_historians_man_in_cave_video_was/
But to actually answer your question, it takes time to prove (or even notice) when a work has been plagiarized, particularly when the person who did wrong does not mention, or intentionally hides the original source. The Hbomberguy video is about exploring that in depth and the IH video is just one example, not the main topic.
Thanks for that link, really appreciate it, and for explaining what that Hbomberguy video was actually about and that IH was only an example.