Yes, that is correct.
The sub claim for a user returned from the userInfo endpoint returns a unique ID for a user in your Okta org. You can be assured that no other user in your org will have the same id.
this.props.auth.getUser() does what you’re suggesting (pulls out the accessToken, then makes the request). this.props.auth.getUser()doesn’t take any arguments.
The sub field is unique, so a simple this.props.auth.getUser() should return what you expect.