No worries. The Fly.io docs do say they recommend 8080 in many places, so I should have realised.
I just wanted to add this here so that if someone else starts searching for it they can find the answer.
Here’s the kind of errors you will see in your log:
[warn] Failed to proxy HTTP request (error: no known healthy instances found for route tcp/443. (hint: is your app shutdown? is there an ongoing deployment with a volume or using the 'immediate' strategy? if not, this could be a delayed state issue)). Retrying in 1000 ms (attempt 80)
[error] instance refused connection. is your app listening on 0.0.0.0:8080? make sure it is not only listening on 127.0.0.1 (hint: look at your startup logs, servers often print the address they are listening on)
See also here for fixing it in reverse:
https://community.fly.io/t/remix-integration-sets-up-the-wrong-port-by-default/8396