[mef51]

No, the measurements are of the redshift of hydrogen in the galaxy so it's talking about the orbital speeds. Also this quote from the press release: "if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”[1]

[1]: https://www.icrar.org/cosmic-clocks/

[teamonkey]

It's a bit more abstract than that. The finding is actually that there's an approximately linear relationship between the measured distance of the outermost edge of a galaxy from its centre, and the average linear velocity of objects at that distance.

Define a rotating circle based on those two measurements, for each galaxy. These hypothetical circles will all have similar angular velocities no matter how big they are, hence they all take roughly the same amount of time to make one rotation.