I would say that reading for free online is different from offering a free eBook as the title indicates. 'Free online HTML book' would seem like a better description to me if that's the case.
That's certainly more descriptive. I always think of those as eBooks though. In my mind I don't really see there being a difference. HTML is one, of many, eBook formats.
The main difference for me would be that I'd consider an eBook to be something I can download (epub, mobi, pdf etc...) and read on my Kobo offline. Can't see how I could easily do that with the online HTML version.