They’re the ones who did the restorations, around 10 years ago IIRC.
It hasn’t been detailed here yet, but he took three photos (on glass plates) through glass color filters (R, G, B) in quick succession on a single camera.
Fortunately, those plate negatives had archival longevity, so the folks at the L.O.C. were able to scan them at high resolution, clean them up, register them, and color correct them giving these amazing results.
I think I first found out about this on dpreview.com.
BTW, if you look closely, you can sometimes see color fringing due to movement of the people, clouds, water ripples, etc.
Much like the color TV camera shots from the late Apollo missions, and for the same reasons.