I have Xcode 11 beta2 and 10.2.1 installed. Xcode 10.2.1 is selected.
If I run ti build -p ios it will bomb out saying no Xcode's supporting iOS 13.0.
Ultimately, it looks the code to auto-select the iOS SDK version when unspecified is not always sorting the Xcode's to prefer to selected one. Specifically if a 2 member array, the custom sort only checks the second Xcode to see if it's selected, not the first. This can result in it not getting "priority" and Xcode's get sorted by version instead (preferring Xcode 11 and iOS 13)