Yes, the arXiv started with PDF-less tex. (I believe people just compiled to postscripts files, which could be printed directly.) But when PDFs appeared, and especially as latex distributions became less standardized, there would have been a natural pressure to accept PDFs.

yes, this is why they have accepted PDF for some time https://arxiv.org/help/submit_pdf

(maybe for as long as since 2004 -- but ironically archive.org could not access arXiv for some years https://web.archive.org/web/20041101000000*/https://arxiv.or... )

Most other preprint services, such as those based on eprints (and in other disciplines) have always accepted PDF.

The reason they accept PDF is not because of the difficulty in getting tex files to compile on different distributions that I mentioned. Indeed, as they say on the page you link to, PDF files created from a tex file are specifically rejected by the arXiv.

Rather, PDF files are accepted for the (fairly small minority) of papers that are written using alternative editors like MS Word.