It would be less confusing if for new multisite deploys the default WP install screen would show in the browser like it does for regular sites (non-multisite).
Trellis figures people will vary in how they want to install WP (manual in browser, manual via wp-cli, some script, etc.), so it leaves the install in your hands. I suspect it would be valuable if Trellis had a pre-built install-on-deploy option (disabled-by-default).