I don't think iframes really do it either, but even if they do nobody uses them that way and a replacement seems okay. It's not really a "competing standard" if nobody wanted to use it.
If I have to pick between standard slow-moving HTML and wild west HTML-reimplemented-in-React, I will choose the former every time. I will never have to worry about "my <p> has been abandoned by its maintainer and is now malware!".
Relatedly, this follows the Notion model, where the UI is intrinsically tied to and limited by the data model. This may have appeal to developers who already are primed to think that way, but it's a terrible UI paradigm for everyone else.
The naming, given 2022’s technology landscape, is also super confusing. Block… no chain, but is it related? Or is this the payment platform formerly known as Square and they released some protocol???
Nope - It basically a React component library/package manager - ReactComponentHub. I do hope it works well, but like many, doubtful from previous examples in the past.