All modern WiFi APs require closed firmware blobs that run below or parallel to OpenWRT.
You replacing the router OS with OpenWRT does nothing when the radio has full DMA access and runs its own OS on its own processor. The OpenWRT layer will have no idea what it's running/infiltrating/exfiltrating.
I say this as someone who has been running and building OpenWRT forever. It's great but it isn't a panacea.