Elasticsearch immutable


I try to understand immutable in Elasticsearch, and read the article Updating API and Partial Update API , but still not clear about it.
The article said Documents in Elasticsearch are immutable, but the Update API mentioned that

Elasticsearch has marked the old document as deleted and added an entirely new document. The old version of the document doesn’t disappear immediately, although you won’t be able to access it. Elasticsearch cleans up deleted documents in the background as you continue to index more data.

According to my understanding, it said the original document will be delete and the update will be add into a new document.
It will be delete if an updating API occur, why it still an immutable document?

Thanks for any reply.

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