The first might be just the time it occupies on the wire (although it'd be a 400G network, that's not quite "commodity" yet IMHO...), vs roundtrip being transfer time, time through switches, time through network stacks, ... to send a packet and reply to it as fast as possible.
But even if this is assumed to be the time to transfer a UDP packet from one network card buffer to another network card buffer directly connected by a cable this seem extremely low.
"on the wire" as in what you'd see if you'd connect an oscilloscope to the "wire" (which would be fiberoptics at that speed...) and watched how long it took from packet start to end. That could work, but it could also just be an error on the page.
Send 2000 byte over network = 44ns Roundtrip in same datacenter = 500000 ns
Isn't a rountrip simply a send from A to B and then a send from node B to node A?