Most likely sending pseudorandom data so that the data can be validated at the other end.

Given they say it's really 19 fibers in one, that's really just 6,600Gb/s per fiber which is really just 4 colors per fiber with one of those and some amplifiers: https://www.fs.com/c/1.6t-osfp-infiniband-1392

Apparently those go into a watercooled switch. Those 1.6T NICs sound absolutely insane. Makes your home 10G network look strings and cans.

It's not that insane in perspective. Probably still needs a whole rack of equipment to run just that test, but the technology is not too far off that it's quite plausible.