Is this kinda like the SPIM system?

Yes, kind of. From what I could gather from the supplementary info in the nature paper (and the patent), there is a cylindrical lens in the excitation path, which creates an oblique sheet that can be moved through the sample.

The return "non descanned" light path uses the neighbouring mirror in the polygonal rotating mirror, and is filtered for the emission wavelength. That's the really exciting bit.

The mirror actually isn't rotating but is mounted on a galvanometer, so can quickly vibrate backwards and forwards. So in effect the prototype microscope is only using 2 mirrors. Explained in the supplementary why rotating mirror isn't suitable. Too slow? Interesting, would've thought that at 15KRPM using a BLDC motor, the scan time would be fast enough: 15K / 12 > 1000 volumes / sec acquisition (scanning done with a single mirror) or 1000/nslice using all the mirrors. For nslice < 100, still > 10 volumes / sec. The 15KRPM figure I have in mind might be way off, which would explain the use of a galvo.

Maybe barcode scanners would be a good source of cheap rotating mirrors. One of these? (images half way down). There's a Hall Effect sensor on the BLDC motor which may be used as a reference for triggering the camera / laser shutter. Then within (360/12) degrees decide how to space your triggers for slicing the volume of interest.

Kugler Precision may have some off-the-shelf mirrors for sale already.

Problem with using laser printer mirrors is that they are not high enough to accommodate the pupil diameter.

Cool design in any case. And if the big boys start licensing the tech, we'll see some even cooler improvements to this working prototype.