The numbers include total application latency, not just the latency introduced by the network, so the improvement to the network latency is larger. As such, applications that are more sensitive to network latency would show larger improvements.
Thanks, Ian
Disclosure: I authored the post
Another benefit that isn't measured in this post but has been mentioned elsewhere in the comments here is that the experience on flaky wireless connections (without changing IPs) should be much better. TCP was designed on the premise that packet loss almost never due to physical failure to transmit a packet, and almost always due to routers queues being full (i.e. network congestion). Wireless networks violate this assumption badly: physical-layer issues are the most likely cause of packet loss on a wireless connection. TCP reacts to wifi packet drops by backing off, assuming that some router is overloaded, but the routers are fine — it's just last hop signal that's bad. In those circumstances, the client should just try again instead of throttling the connection to nothing. Since HTTP/3 uses UDP, it can potentially handle dropped packets more appropriately.
On your second point, it's configurable how TCP reacts to packet loss. For example, wasn't BBR congestion control made to address exactly that case?
Switching between cells will cause a latency spike, and a lot of correlated packet loss if the subscriber has a large queue of undelivered data, but that's all. But handling packet loss and ordering are what TCP is supposed to do.
I have no idea of what you mean by "the TCP connection is no longer there". The TCP connection is a distributed system living on the client and the server. It doesn't go away unless one of the endpoints decides so. (Modulo stateful middleboxes, like NATs. But nobody in their right mind would run a TCP state-aware middlebox on a cellular network base station).
IP changes are relevant when switching networks entirely, like going from WiFi to mobile.
One type of roaming that does trigger for smartphones and similar devices is WiFi->4G/5G->WiFi.
You're at home, obviously you don't want expensive mobile network data charges when you've got WiFi. So the connection is over WiFi. But as you walk out the door, currently your application software needs to spot that the WiFi is going away (not too hard), connect over the mobile network (unless your policies say to give up instead to save money) and keep going. QUIC would allow this to be done transparently at the transport layer, at least in some cases. When you reach a coffee shop/ friend's place/ work and there's WiFi again, the opposite transition saves you money and if you go indoors and signal is weaker may also be necessary to deliver a working network.