ah! finally figured it out - though not exactly the way I wanted.
looks like HomeKit does support light level sensor - but Home app doesn't in iOS 12 (don't know if new iOS 13 will add support for it). so I had to use "Eve for HomeKit" app (https://apps.apple.com/us/app/eve-for-homekit/id917695792) - it is an extension app on top of Home app. probably other extension apps can do it too - but did not try.
I am using Philips Hue motion sensor. when I added it to HomeKit, HomeKit added 3 sensors - motion, temperature and light level. using that light level sensor, I was able to create automation in Eve app to switch on lights whenever light level will go down to certain limit. and it worked! Home app shows that automation but doesn't able to show the trigger condition.
two issues with this automation -
0) looks like Philips motion sensor updates light level value only when it senses motion. so this automation doesn't light on even if room is dark - it waits for a motion to trigger. so it, kind of, works as long as I move around in room (also doesn't light on if room has enough light).
1) what I observed, it works only when light level crosses over the threshold lux value I set. I set it to 20 lux. it works whenever light level goes down from 21 lux (or higher) to 19 lux (or lower). but it does not trigger if light level is already below 20 lux to start with.
not sure how to overcome these two issues.