"...reuse of public domain is material is not."
Uh? No doubt you're correct but from what I have experienced I'm still not clear about the details.
It's years since I've posted anything on YouTube but there was one instance where I received a copyright infringement notice from YouTube. The 'infringing' material involved was a 78 audio recording over 80 years old that was in the public domain which I'd downloaded from the Internet Archive then posted to YouTube. The only reason I did this was because there were many of the artist's recordings already on YouTube and I posted it to help complete the artist's collection.
Much later YouTube sent me a somewhat ambiguous note to the effect that there had been a copyright complaint but it was not taking any action however I could lodge an objection in case the matter escalated (it may not have used those exact words but that was my understanding of the matter). I chose to ignore it.
In my post I made it clear† where the recording had originated from and whilst YouTube would not allow me to post its download URL as plaintext I embedded it into the accompaning image where it was fully legible.
That was some years back and the recording is still there. Thus YouTube was well aware it was a public domain recording but did nothing about it. Those who posted the other recordings must have had a similar experience.
Also, there are hundreds of old documentaries, newsreel footage etc. on YouTube that are in the public domain that havn't been taken down. If this is a violation then why hasn't YouTube taken action to remove them?
† Edit: here's the link: https://m.youtube.com/watch?v=yWoEanmuolQ