>He must have worked with a company that can do the surface mount assembly

He did (there are centroid files in the production folder, which tell the board house where to put the components), but you'd be surprised at how possible it is to assemble something like this by hand. You won't believe me, but I find it easier than through-hole soldering (because you don't have to keep flipping the board over).

But there's a 99.9% chance this was done in-house at JLC or PCBWay.

Indeed, JLC for this one, though I did start off by hand assembling the first version with a 3D printed jig to locate the LEDs.