Well, curiosity got the better of me so I carried on investigating. Twrp and magisk can indeed be preserved throughout the OTA process as described in the guide linked to above.
I started with a clean slate: eOS 0.14q. Download and install OTA eOS 0.15q and reboot.
Install twrp to device by flashing the installer zip, reboot device. Check twrp recovery present ,ok.
Install Magisk with twrp, reboot and finish setup on device by touching Magisk app and following prompts.
(Note: Magisk v22.1 would not complete the on device set up. I uninstalled it and tried v22.0 which did work. Updated it in the app to 22.1)
This is where the experiment started and I followed the guide fairly closely. A bit of interpretation is needed because the screenshots aren’t exactly up to date, but the steps are the same.
Downloaded OTA eOS 0.16Q (settings in the updater set to ‘priority’ to test the speed) but don’t select ‘install’.
Step 2 of the guide: In Magisk select ‘uninstall magisk’, ‘restore images’. Do not reboot.
Step 3 : Back to system updater, install the OTA, do not reboot when the prompt arrives.
Step 4: Magisk again. In the modules window search for the twrp a/b retention script, install it, but don’t reboot when the prompt arrives.
Step 5: Go back to Magisk Home, select ‘install magisk’ and ‘install to inactive slot’ and ‘Let’s Go’ . After flashing has finished Reboot when prompted. That’s it. Everything is there as it should be; magisk and twrp on device.
Downloading OTA through to the last reboot into eOS took 30 mins; so the priority setting speeds things up.