But "opt in" and "opt out" as verbs mean "choose to/not to participate". So you can in fact "opt out" of an "opt-in" feature. It means choosing to keep the default of not opting in.
You could rephrase the 23andme statement to be something like “users of this feature can choose from a range of privacy settings. For complete privacy, do not use this feature (which is the default setting).”
Anyway, this strong social expectation to vote is the only reason you can even remotely say that you are "opting-out" of voting.
Any other case, where a decision to be made only exists really as a feature in the UI, there is a much tighter relation between the implementation of the UI feature being by default on (thus "opt-out") and the way that we speak about it being "opt-out". If thus, by their own admission, there are things to opt-out of, then this is by definition an opt-out feature. Claiming the opposite is disingenuous.