That might be a little strong in light of this cloud being "one of the few such clouds large enough for us to be able measure its magnetic field.” They seem unsure if it will survive impact with even the fringes of the galaxy.
Astronomers think origins of so-called high velocity clouds are mixed, some stemming from burst bubbles in the gas of our Galaxy, some being primordial gas, and some associated with small galaxies our Galaxy’s gravity is shredding from a distance.
I think another source possibility might be plasma jets from accretion disks.
My point is, they don’t even need to be able to measure it to know it has a magnetic field. Simple deductive reasoning should tell them that. It’s a plasma. Plasmas are ionized conductors. If they have ever moved anywhere in proximity to a magnetic field, which they must have, since they didn’t originate from a vacuum, then they will have received an electric current due to the principle of induction. A moving electric current produces a magnetic field. QED, a plasma moving in space will have a magnetic field.
That’s a bit simplified, since plasma mechanics actually details how that magnetic field will evolve to contain, shape, and affect the motion of the plasma in different ways as well. Astrophysicists just aren’t generally very interested in thinking about this stuff, even though they know the entire universe is literally filled with plasmas.