My sign-up e-mail said the following:
Now you can easily access your e-card through Garuda Indonesia Mobile Apps.
With the new e-Card you now have the convenience of using the digital version of your GarudaMiles membership card directly from your smartphone.
The e-Card can be used for all GarudaMiles identification and transaction purposes at the Garuda Indonesia Sales Office, check-in counters, airport lounges, and partner merchants.
Makes me wonder if the e-Card would suffice assuming the membership card is required.