How to access web dev servers running on localhost with non-standard ports from the network?
This may be considered a duplicate question as I arrived at the solution from this answer.. I didn't give up searching, sorry about that.

To help others I'll mention what I got wrong.

I didn't have to change any firewall or sharing settings - it was to do with where the application was being hosted. I assumed that when I viewed 192.168.0.164 from the other Mac, it'd magically map to the host's 127.0.0.1... that is not the case!

So instead I explicitly hosted the application on 192.168.0.164 instead of 127.0.0.1 and it worked fine!