Except the simplified versions are made by humans who can preserve the flavor of the language and the important meanings, unlike this tool which is like replacing the Mona Lisa with text that says "woman sitting".

Unfortunately that popularity directly translates to the AIs ability to digest and paraphrase a book. LLMs have been trained on what is available in computer text format, which means mostly internet sources. English has an outsized presence on the internet compared the to actual number of native speakers, so there's magnitudes more training data for it than any other language. The models of other languages will be severely limited, if AI companies have spent the resources to train them at all.