With the single-packet attack, you look at the order that the responses arrive in, instead of the time they take to arrive. Since the responses are on a single TLS stream, they always arrive at the client in the order that the server issued them in. Hope that makes sense!

I take them to be asking why jitter on the return path doesn't confound the results, regardless of the trick used to ensure they arrive concurrently (and cancel out the jitter on the ingress path). The responses to single-packet H2 attacks are not themselves single packets.