The "narrow waist" concept was originally that the only protocol that the internet required every network to speak natively or directly was IP. TCP and UDP were intended to be two protocols among many at the next layer up, but through various factors and accidents things got ossified to just those two protocols at that layer and even that so much so that increasingly in the last few decades TCP/IP is seen as the "real internet" (the "waist" expanded).
From that perspective QUIC looks like a hack: it's built at the next layer up (on top of UDP rather than a true peer among UDP and TCP) because building at the right layer level is "no longer allowed" (thanks to decades of firewalls, middleboxes, routers with bad assumptions baked in, etc).
Those things that ossified TCP, though, are also many of the reasons that QUIC can't compete with TCP and that TCP is inherently special on the modern internet as one of only two "approved" protocols and something of a "last man standing" even among them. As an example, pointed to in that RFC list, QUIC has to define its own congestion control, and it's something of an application-level service. TCP has decades of congestion control tech in the wild, some of it baked incredibly low level into routers, so much so that a lot of it is just "automatic" and "magic". Applications don't need to do any work to opt-in to it. It's sad that that work is hard-coded specifically to TCP and only TCP and cannot be easily applied to new network protocols, but that's just the reality of the modern internet.