Hello Developer,
Yes, you are right the third parameter in store device token method take value as a string. But in order to get the exact value of platform from client, we have created one enum class and according to device type developers can choose their platform. For example: In documentation DeviceType.ANDROID is used for Android device.
I believe it will answer your concern, if you need further help regarding this. Please let us know, we are happy to help you.
Regards,
Himanshu Sharma