As someone who has supported multiple sites running UniFi APs... They can act as a repeater bridge for WiFi. (If one AP loses an uplink, but still sees another AP that has an uplink, it'll retransmit the traffic it receives to the next nearest good AP wirelessly.)
They do have a passthrough port, and you might be able to use them as a full bridge, but I wouldn't recommend it. You'd still need a UniFi VM or Dream Machine or whatever to run as the controller for the APs. And that is just to get it running in the first place. Once you do that, and place the APs, the remote end might be difficult to recover or reprogram without an available hardwire uplink that can see the controller. (Like the AP without a current uplink wouldn't recover properly after a power outage. You'd usually have to power cycle it again after the good AP came up. And if you make a configuration update, it doesn't always get pushed to the AP without an uplink. It was really easy to orphan it. I also never found a good way to see statistics on packet loss across the bridge between the APs, etc.)
I always considered an AP with a broken uplink as just sort of running in an impaired mode until maintenance could run a new cable.
