epub is an HTML based format. Essentially it is an archive file of HTML and metadata. So the issue isn't so much whether it is better than HTML but more about some of the tools available. Most ebook reader applications are designed to optimize reading of books - they usually make moving back/forward in 'pages' easier, have consistent access to table of contents, allow easy setting of font types, sizes and color etc. For example, I find epub works far better with VoiceOver (the text-to-speech application in macOS, ipdaOS and IOS) on macOS, ipadOS and IOS than does either Safari (for HTML) or PDF (with any of the available PDF viewers like Preview or Adobe etc) for reading books. Likewise, the Kindle app on macOS, ipadOS or IOS works really well with voiceOver. The web based Kindle app is not accessible at all.
As Raman points out, texinfo under Emacs and .info files are very accessible thanks to packages like his Emacspeak or others like speechd.el. However, other formats generated from Texinfo sources are less so. For PDF the issue is that TeX/Latex PDF generation tools are very poor at creating accessible PDF documents (this has been acknowledge within the TeX/Latex community and there has been some work to improve the situation, but the changes required are non-trivial and progress is slow).
It has been some time since I looked at HTML documents generated from texinfo, but when I last checked, the accessibility was not great (accessibility to documents is not a binary proposition - some documents are sort of accessible, but lack appropriate tags/labels to make navigation of the documents 'make sense', so you might be able to get parts of the document read out, but being able to effectively navigate the document and find what you want is difficult or the data is presented 'out of sequence' compared to how a sighted person would see it).
For me, the advantage of having Emacs documentation in one of these other ebook formats is that it could make it possible to access this documentation outside of Emacs. I find this quite useful. For example, when working on a project, I might open reference books on my ipad or iphone while working in Emacs on macOS or GNU Linux. I find it is sometimes more convenient to check things on the ipad rather than switching buffers/frames under Emacs. I can also use apps that will read epub (or other formats) continuously, which is handy when doing boring tasks like hanging out the washing - Today, I was listening to 'Elements of Clojure' while hanging out the washing.
the point of my earlier post was that the generation of documents in other formats, like an ebook, from texinfo sources is unlikely to be that accessible without a fair bit of effort. I have had good success with Calibre for other document types, but it did take a fair bit of tweaking and experimentation. Having the Emacs documentation in ebook formats would likely be useful to many people, but getting a decent conversion may be challenging and getting an accessible version even more so. I do think it is important to be able to generate the output from existing texinfo files i.e. don't want to have 2 versions of the documents as we will never keep them 'in sync'.