varnas is present in the earliest of Vedic hymns

Which are, to my understanding, apaurusheyatva. But yes, on further research you are right to point out the status of the Manusmiriti.

I have just been describing some facts that are fairly known amongst students of the subject.

Nevertheless, we surely do admit the existence of very conservative Hindus who believe an iron-clad caste society is justified by the scriptures.

(Although, as an aside, Hinduism largely does not seem to suffer the same sort sort of religious fundamentalism which plagues the Abrahamic faiths).

Where have you heard the claim that all Hindu holy books - including the smritis - are eternal?

I haven't and I'm not claiming that. There are some books which are "unauthored" and those which are subject to temporal variation.