From the article; the HTTP Request header that results in upgrade to HTTP/3:

  alt-svc: h3=":443", h2=":443"

> The HTTP Alt-Svc header it received in packet 25 included the directive h3=":443", so when we then reloaded the page (note: not shift-reload, which would have caused Chrome to "forget" the Alt-Svc for this site), Chrome could switch over to QUIC (packets 31 onwards) and then make the request using HTTP/3 (packets 44-45).

HTTP/3: https://en.wikipedia.org/wiki/HTTP/3