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It's not that both parents play golf.
Literal
Parents both golf [object-を] do [it's-not-the-case-that-のではない].
~のではない is a partial negation pattern — 'it's not that X.' Functionally similar to ~わけではない but slightly more direct. Here it clarifies that only one parent plays golf, not both — partial negation hedging the universal scope of 両方とも.