Is it "weird" that all disc galaxies rotate once every billion years? In a recent paper (Cosmic clocks: A Tight Radius - Velocity Relationship for
HI-Selected Galaxies by Meurer, et al.), it was noted in the conclusion that:


  [This] implies a constant orbital time of ∼1 Gyr at [the outermost] radius [of disc galaxies].


Given what we knew about disc galaxies and dark matter up to this point, is this an unexpected conclusion? My sense is that it is strange that all disc galaxies have a constant rotation regardless of their size, but my intuition may be wrong. I know that dark matter speeds up the rotation of the outer parts of disc galaxies, but isn't it strange that they converge on this value?
It is not weird.

The comment is about a certain class of galaxy, and not just a class but a stage of galaxy formation and development within that class.

We start with a fairly constant distribution of matter within the universe.

Stars for when this matter clumps together. This clumping is going to have a fairly even distribution.

Stars then clump together to form structures that we identify as galaxies.

The distribution of this clumping is also going to have a fairly constant distribution.

This carries on up the chain to clusters of galaxies, and clusters of cluster and so on.

Going back to stars, we identify main sequence stars. These are stars that start of with a mass and composition that is very common and then have common life cycles. They take so long to burn up certain percentages of their hydrogen, so long burning combinations of hydrogen and helium and then so long going up the periodic table forming lighter elements and in the process becoming the different types of stars we define as being on the main sequence.

For galaxies it is similar.

Spiral galaxies need to be of a certain size and angular momentum to generate their signature spiral structure. They did not start out as spiral galaxies. And they will not remain spiral galaxies. They are simply how galaxies of a certain size and angular moments appear at certain stages in their development.

This means that the outer edge angular velocities of these types of galaxies (for a specific definition of their outer edge) are going to be similar.  

For some information on the development of spiral galaxies and other galaxy types:

https://astronomy.swin.edu.au/cosmos/S/Spiral+Galaxy+Formation