Bonus round! The back of the Watch4 Classic motherboard doesn't give us anything to work with, but the Watch4 shows us a couple more of those same chips:

S915 intermediate frequency IC

Broadcom BCM43013 ultra low power dual-band 802.11n Wi-Fi and Bluetooth 5.0 combo IC

We'd guessed from the data sheets that these watches are similar, and well … they are. Silicon-wise, there doesn't seem to be anything differentiating the two.