A laser pointer at 2B FPS [video]

How is the image focused and not a big blur?

Shere are so many levels this could be answered at.

All light in a narrow cone extending from the camera gets recorded to one pixel, entirely independently from other pixels. There's no reason this would be blurry. Blur is an artifact created by lenses when multiple pixels interact.

There is a lens in the apparatus, which is used to project the image from the mirror onto the pinhole, but it is configured so the plane of the laser is in focus at the pinhole.

What I don't understand is how the projection remains in focus as the mirror sweeps to the side, but perhaps the change in focus is so small.

Depth of field, small aperture

It’s in focus because only a small angular area from the scene contributes to the light that reaches the sensor. You get blur when sensor elements (individual pixels) receive light from too wide of an area.