The open source driver receives and forwards the mesh information element to the proprietary firmware. It is also capable of transmitting this information to userspace. Thus, to iw or hostapd it makes it look like the device is capable of running mesh point. However the real processing is done in the closed source firmware, the binary files in /lib/firmware/mwlwifi/. There, the information gets blackholed and discarded because the firmware doesn't know what to do with it - it is not coded to support it. There once was plans, but now there's none - as alluded to by @anomeome.
In fact, I was able to "enable" 802.11k (Assisted Roaming) support: WRT AC series: What information elements are used for 802.11k - RRM?
But I'm not sure if the neighbor reports are being sent out, nor have I ever seen them in wireshark pcaps.