It is supported, our installation instructions are simply configured for root directory. Here's what you do for a subdirectory:
location /flarum/ { try_files $uri $uri/ /flarum/index.php?$query_string; }
location /flarum/api { try_files $uri $uri/ /flarum/api.php?$query_string; }
location /flarum/admin { try_files $uri $uri/ /flarum/admin.php?$query_string; }