That diagram you show is from page 32 (https://pk.fox-ess.com/download/upfiles/EN-H1AC1(G2)-Manual-V1.6.pdf), for On-Grid Parallel Connection/Off-Grid Parallel Connection to connect multiple inverters together, consistent with what your supplier has told you. Is the diagram on page 23 and reference on page 26 more relevant? I agree, it doesn’t seem entirely clear. Your inverter installer should know, or refer back to Fox technical support.
Is this the WaveShare unit? https://www.waveshare.com/wiki/RS232/485/422_TO_POE_ETH_(B) As a comfort, it is isolated for both signal and power, so don’t expect any smoke clouds!
Further searching:
Looking at this page: GitHub - nathanmarlor/foxess_modbus: FoxESS inverter integration. Connect directly to your FoxESS inverter (no cloud!) for real-time status and control. and following the links to Home · nathanmarlor/foxess_modbus Wiki · GitHub and Waveshare RS485 to ETH (B) · nathanmarlor/foxess_modbus Wiki · GitHub and Modbus Wiring Guide · nathanmarlor/foxess_modbus Wiki · GitHub and cross-referencing with FoxESS to Home Assistant Integration Options (H1/AC1&AIO) - FoxESS Community - Owners & Installers Forum would tend to point to page 23 pinouts in the manual as being the appropriate ones (pin 3=485B and pin 4=485A, regardless if 8P or 16P connectors). See the connection discussion in these forums too at Foxess Realtime Monitoring
The integration for HACS at GitHub - nathanmarlor/foxess_modbus: FoxESS inverter integration. Connect directly to your FoxESS inverter (no cloud!) for real-time status and control. and also Protocol Wizard part 1. No more YAML! Run-time addition and control of devices and registers on the fly may be something to research also.