PC Master Race

That is not what plagiarism means

Oxford English Dictionary

The action or practice of taking someone else's work, idea, etc., and passing it off as one's own

Merriam-Webster

to steal and pass off (the ideas or words of another) as one's own : use (another's production) without crediting the source

Dictionary.com

an act or instance of using or closely imitating the language and thoughts of another author without authorization and the representation of that author's work as one's own, as by not crediting the original author

All three definitions clearly state that Plagiarism is taking some production of someone else's and claiming it as your own. That there is some kind of deception going on as to who created the original thought/work. Merriam-Webster's definition has that second component talking about the act of using without crediting the source, which LMG didn't do at first but later added a pinned comment. While not immediate and the barest amount of effort on LMGs part, but it still is credit.

Plagiarism has no legal component to its definition.

Copyright does have legal implications as it is someone's right to duplicate a work. In general a creator of a work has exclusive rights to reproduce it, but there are exceptions (everyone's favorite Fair Use laws). With LMG being Canadian the legal side is more complicated but in US courts it's been tested that one such exception is around additional commentary and that the usage of the work was limited as to what was relevant to being actively discussed (big case here being H3H3 a few years back). Even by GN's own admission the WAN show was taking phrases and repeating them verbatim, but just that, only phrases. Ones pertaining directly to the on hand topic of EVGAs ending partnership with Nvidia. They were not showing GNs video, reading his script word for word start to finish. Again, IANAL but I find it highly unlikely that a US or Canadian court would say that what LMG did on WAN Show meets the definition of a copyright violation

Edit:

And to answer your last point directly, Plagiarism and Copyright are orthogonal to each other. You could plagiarize by not giving public credit but still get copy permission from the copyright holder. Semantically kinda weird to think about