You’re not doing anything wrong. The Ionic-CLI tool uses cordova under the hood, and the Cordova-CLI uses ios-sim under the hood by running the /platforms/ios/cordova/run shell script.
If you look inside that script, you’ll notice that it exposes the --target switch which maps to --family for ios-sim.
A look inside the code for cordova emulate shows that it concatenates additional command line options before invoking the above mentioned script.
That means that running cordova emulate ios --target=ipad is what you were looking for.
Cheers.