Environment variables set in a rose app file are used when you use rose task-run to execute the task. Rose will set up the environment variables, create files, then run the command specified in the [command] section of the config file.
You’ve not specified [command] in your file, so I assume you’re not running with rose task-run?
No, I’m trying to really understand rose/cylc but I don’t find more than the most cursory of documentation. The tutorials on the metoffice’s website are fine, but they don’t go into that much detail. There’s a cylc documentation but it doesn’t talk about rose at all.