"and Pony Express" is the operative phrase. As the paper states on page 1, "Snap is deployed to over half of our fleet of machines and supports the needs of numerous teams." According to the paper it is not niche.
An example of how Linux TCP is unsuitable for datacenters is that the minimum RTO is hard-coded to 200ms, which is essentially forever. People have been trying to land better or at least more configurable parameters upstream for decades. I am hardly the first person to point out the deficiencies. Google presented tuning Linux for datacenter applications at LPC 2022, and their deck has barely changed in 15 years.
Later
Also, the idea that QUIC is a concession made to intractable Linux stack problems (the subtext I got from that comment) seems pretty off, since the problems QUIC addresses (HOLB, &c) are old, well known, and were the subject of previous attempts at new transports (SCTP, notably).