From looking at TFA what I garner is that galaxies got arms a long time ago "as they got older". Specifically: "Then the clumps elsewhere in the glalxy (sic) begin to smear out forming woolly, indistinct arms." Nailing down the process, they follow with "More complex structures follow later." Wow, mystery solved.
Now I seem to remember some research 10 or so years ago that showed through computer simulations that 2 galaxies composed of homogeneous distributions of stars, if they pass relatively close to each other at speed, will affect each other and cause each one to form spiral arms. That made intuitive sense, backed up with data. This article... really... what does this article say?