No, it's definitely not duplicitous, and is standard practice across the industry.
It boils down to "we're not claiming to own the rights to your content -- you still retain those -- but we need a grant of permission from you to ensure that we can publish it on our own site without facing possible liability".
This isn't specifically an issue with this service alone (I like it), but the approach to UGC (user generated content) in general.
What's unclear is what rights or license remain if someone deletes their account and content. It's trivial to clarify, making omission is a decision.
Use of the word "improving" is pretty general and broad, and can be about the priorities of the vendor over the customer.
What's missing is the clause that closes the loop and doesn't give them a lifetime license.
"you grant Supertone a worldwide, non-exclusive, royalty-free license to use, reproduce, adapt, and display content solely for the purpose of operating and improving Supertone Shift."
Its open-ended, yes, but that does not imply any duplicity. Platform providers have an incentive to minimize their liability exposure, so they're not going to volunteer to add language that applies additional qualifications and exceptions, but that does not in itself imply that any deception or abuse is going on.
It's certainly proper to call out examples of actual bad behavior by specific organizations, but not so much to treat defensive boilerplate as though it is itself bad behavior.
It boils down to "we're not claiming to own the rights to your content -- you still retain those -- but we need a grant of permission from you to ensure that we can publish it on our own site without facing possible liability".