I don't think it would be legitimate for them to penalise you. The whole point of that recruitment tool is to help disadvantaged candidates where they can. Such as a candidate who completed A levels at a low performing school for example. If it doesn't apply to you, there won't be much use of the system in their reviewal of your application. In essence, it is not assessed in the way the rest of your application is. It is, for one way to think about, mitigating circumstances for whom it benefits. You have every right not to complete it, as shown by the option. Rejecting you on the basis that they gave you an option to not provide information, and you choose not to, is deceitful as it has not been indicated as an essential/required assessment point. Also, it would be a highly questionable practice.
Bit of a stretch here but rejecting someone because they aren't disadvantaged runs the risk of being discriminatory - this does not mean events and insights towards social mobility and DEI are before an argument about that develops...