If they add easy support in NGINX and HTTPD, then its easier for self-hosted endpoints to change as well, with minimal to no effort on their side.
The way I have understood, the book says that what is now in use (these 7%) is a "Google-only-QUIC" whereas the "standardized HTTP/3" is still used... nowhere?
The IETF QUIC remains a work in progress, perhaps to be published in 2019. HTTP/3 is an application layer on top of (IETF) QUIC, it might also be published in 2019 or later. There are implementations of current drafts, and the rough shape is settled but they're a long way from being truly set in stone and aren't in anything ordinary people use.
So unsurprisingly nobody is already doing a thing that isn't even standardised yet, but people are, as you see, writing about it.
"> The book says 7% of all internet traffic already uses QUIC (HTTP/3)"
was wrong: the book doesn't say that, and that what is claimed that the book says (even if it doesn't) is false in more aspects.
But this book isn’t about concerning yourself with using it or implementing it, it’s about understanding what the future holds, how it works, and what roadblocks lie ahead.
The lack of API support in OpenSSL for it’s TLS requirements and poor optimization for heavy UDP traffic loads on Linux et al (they say it doubles CPU vs HTTP/2 for the same traffic) sounds like it’s going to be a major hurdle for widespread adoption any time soon.