I don't know what twitter you've been reading but I wouldn't call what I see "knowledge".
I assume they are referring to the fact that books older than about 70 years are in the public domain. The rest are protected by copyright and not free.
I think so too, but I am referring to this statement:
> The overwhelming majority of our knowledge is in the public domain
if we talk about biology, absolutely not, but most of high school students won't finish basic physics, so this make sense in some region