He's most likely using the laptop for capturing photos (possibly tethered capture), and doing post-processing on the PC. Post-processing is what requires high performance hardware. It's also likely that he already had the Macbook and didn't feel the need to replace it at the same time.

I'm in the same boat, as it turns out. I switched from macOS to Windows for post-processing this summer, but I still have my Macbook for image capture and driving a telescope in the field.