They’re talking about disk galaxies. The Milky Way is believed to be a barred spiral galaxy, not a disk. Lacking the ability to see the whole thing from a distance of course, it’s somewhat of an educated guess.
(This is not helped by the fact that their picture is of a spiral galaxy, not a disk.)
Disk galaxies come in two flavors: Spiral and lenticular. So spirals are included in the discovery.
I believe that all galaxies are essentilly discs. The disagreement would be argument over the thivkness of the accretion.
The rotational period is a function of the gravitational friction of the black hole