I've read almost all (or maybe all) of Agatha Chrstie's books published under her name.

I think the more accurate question would be "Do Christie's books reveal indications of a belief that a particular race or ethnic group is inferior solely because of its race or ethnicity?"

If the use of the n-word indicts Christie, then we should examine Joseph Conrad as well, and many others.

There are probably some signs that some of Christie's novels reveal / promote cultural stereotypes, especially the more "exotic" stories like "Murder in Mesopotamia." But if you compare her "exociticism" in these books with (for example) "Indiana Jones and the Temple of Doom," then Christie is far less prejudicial.

The question is made more complicated because many of the quotations in the article you cite are made by characters who may be revealing their prejudices, not Christie's.

One thing is certain (to an impartial reader): Christie does not reinforce gender stereotypes.

We should not cancel her works. But we should not dismiss them either, without a careful examination of her purpose in discussing such stereotypes.