Imagine a team of archaeologists excavating a megalithic tomb near Malmö. Everyone's old South Scandinavian peasant stock except for one person who was born to Sardinian immigrants.
Only one of the diggers is closely related to the megalith builders: the Sardinian Swede. Because old South Scandinavian peasant stock only arrived with the Corded Ware culture, long after the tomb was built. Sardinia is the main place where the meg builder genetics survive.