The QUIC working group that was established to standardize the protocol within the IETF quickly decided that the QUIC protocol should be able to transfer other protocols than "just" HTTP. Google-QUIC only ever transported HTTP - in practice it transported what was effectively HTTP/2 frames, using the HTTP/2 frame syntax.
It was also stated that IETF-QUIC should base its encryption and security on TLS 1.3 instead of the "custom" approach used by Google-QUIC.
In order to satisfy the send-more-than-HTTP demand, the IETF QUIC protocol architecture was split in two separate layers: the transport QUIC and the "HTTP over QUIC" layer - the latter of which was renamed to HTTP/3 in November 2018.
This layer split, while it may sound innocuous, has caused the IETF-QUIC to differ quite a lot from the original Google-QUIC.
The working group did however soon decide that in order to get the proper focus and ability to deliver QUIC version 1 on time, it would focus on delivering HTTP, leaving non-HTTP transports to later work.
In March 2018 when we started working on this book, the plan was to ship the final specification for QUIC version 1 in November 2018 but this has been postponed a number of times and, at the time of writing (June 2020), is entering final stages.
While the work on IETF-QUIC has progressed, the Google team has incorporated details from the IETF version and has started to slowly progress their version of the protocol towards what the IETF version might become. Google has continued using their version of QUIC in their browser and services.