I don’t think that is true if the Hide email address taken site setting is disabled. But I think you are correct that the error that the email may already be registered is not shown in this case. I wonder why. In this case, it seems to be the correct message.
Because of Make reason for 'error moving posts' visible in the interface - #2 by zogstrip, I thought it might make sense to check the network response in this case too, and tada: errors: [ "param is missing or the value is empty or invalid: invite_code" ] the error is there too.
So maybe the error shown can be improved in a similar way. Though a validation of the field in the frontend, as all the other fields have, would be nice too.