combining a block quote and the source of the quote using the cite element

In order to support publishers in Germany the publisher organisation (Boersenverein) published EPUB3 accessibility guidelines.
https://www.boersenverein.de/beratung-service/barrierefreiheit/leitfaden-barrierefreie-epub3-e-books/
There is also a standing working group to further support publishers when new questions concerning accessibility arises.

We were recently asked about combining a block quote and the source of the quote using the cite element.

In the above guidelines, we recommend the following construction:

„Duis autem vel eum iriuredolor in hendrerit in vulputate velit
esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros
et accumsan et iusto odio dignissim qui blandit.”

(Zitat-Quelle)

The DAISY knowledge base mentions in its FAQ that this is not permitted in HTML.
https://kb.daisy.org/publishing/docs/html/quotes.html

Can you please point us to the part of HTML specification that makes this illegal?
And why should the cite element only contain the title and not the author?