Basically Man in Cave is a rather blatant copy of an article by Mental Floss in 2018, but animated.

TLDR if you don't wanna watch the 20 minute segment:
Internet Historian used the Mental Floss article as a script for the video. The owners of the article made a DMCA takedown on the video as it was used without permission. Internet Historian has downplayed why it was taken down, reworded and removed swaths of the video to make it sound less like the article. Released the re-upload of Man in Cave with another video at the same time in order to try and distract from the story that he did indeed copy a little known article.

He does good work... but he also totally did copy the script of Man in Cave. Hbomberguy does a pretty good argument proving this.