This successfully dumps the app contents and executable BUT DOES NOT DECRYPT THE EXECUTABLE ON A12 DEVICES.
At least whenever I've tried to use it, the executable is a jumbled, encrypted mess in IDA and I can't do anything with it. This basically just takes the contents of the app that you can access on iOS at the previously accessible directory /var/containers/Bundle/Application/*YourAppHere*/ and copies it and bundles it into an ipa for you (which is fine) but it definitely doesn't decrypt anything.
I guess the only hope for reverse engineering apps on iOS 12 for us A12(X) folk is finding someone with a jailbroken, non-A12 device to decrypt for us lol.