Update for anyone having this problem - there was a bugfix added into the Haiku nightly packages, from hrev57615 onwards. See announcement here:
My Lenovo Thinkpad X1 Yoga 3rd Gen (Core i7 8th Gen) comes with the Intel 8265, and I can confirm the built-in WiFi now works for me in Haiku for the first time. I was able to connect to an SSID, enter my password, do web browsing, ping from the terminal etc. In my testing I was using hrev57633 x64 as a Live CD booting from USB stick.