I think the claim that humans stopped evolving is an uncharitable reading.
Another would be, 50k years is such a narrow window that evolution cannot be observed across it.
Another would be, 50k years is such a narrow window that evolution cannot be observed across it.
Your alternative claim is just factually wrong. The population frequency of an allele under selection pressure can change within a few generations.
For example the HbS allele, that confers both sickle cell trait and malaria resistance, became less common among Africans transported as slaves to regions where malaria was not endemic.