I downvoted this, since it's a drive by insult without any kind of support provided.

If plot plagiarism was a crime or even morally reprehensible, half the books in my town library would have to be destroyed.

He was inspired by C. S. Lewis' that hideous strength.

Interesting, I hadn't heard that, do we know that definitively? (THS was the only one of the Space Trilogy I never read, I should correct that.)

Orwell reviewed That Hideous Strength and said it had some good points. But it's definitely the weakest in the trilogy, and definitely did not influence 1984 in any obvious way. Still worth reading, though.

It's a very hard read. It took me several times to get into it.

Having read both I can't speak for where Orwell drew inspiration, but I never noticed any similarities

Plagiarized of what by who?

He may be talking about the underrated Swastika Nights. A new world, after memory of the old one has been erased is part of the plot, as is a secret manuscript.

That said, stories and plots have been retold ever since the beginning of literature. Here's a lovely article about various versions of the well-known story about the Baalshem being asked to intercede for a sick child: https://brill.com/view/journals/jjtp/22/2/article-p127_2.xml

Yevgeny Zamyatin's We. (1921) https://en.wikipedia.org/wiki/We_(novel)

Also Jack London's "The Iron Heel": https://en.wikipedia.org/wiki/The_Iron_Heel

Taking inspiration isn't plagiarism

books of the same genre share motifs and themes

H.G.Wells - The Sleeper Awakens https://en.wikipedia.org/wiki/The_Sleeper_Awakes