Gravity + angular momentum has the emergent behavior of collapsing distributed masses one dimension at a time:

Clouds(3D) -> Discs(2D) -> Rings(1D) -> Point Masses (planets, stars) (0D)

So flat disc phenomena are obviously very common naturally occurring things. Space is jammed wall to wall with discs and rings.

A flat explosion is certainly interesting, it's not too hard to posit some ways they might come about, but it'll be interesting to see what conclusion their data actually suggests.

A star that collapses does in fact retain its angular momentum, so yes, neutron stars spin very fast (pulsars) and black holes are expected to spin at something very close to the speed of light, at least when they initially form - they should gradually dissipate some of that energy as gravity waves over time.