Proves is a strong word... It's a data point to be sure, but it's not proof.
They may have more conservative or liberal tendencies, but to think that that all Justices will vote the way that people want to pigeonhole them into is ridiculous.
I suppose it proves they aren't 100% (not 99.9999%....100%) bound to always make decisions precisely aligned with a political party.
But no reasonable person suggested that.