Continuum fallacy. Might as well claim that there's no such thing as blue or violet, since there's a gradient between them.

Also you can establish homogeneity using genetic analysis such as the fixation index. Unsurprisingly, Swedes and Finns are extremely closely related.

https://en.wikipedia.org/wiki/Fixation_index#Autosomal_genet...

There are many possible metrics to measure heterogeneity, such as linguistic and religious diversity, variations in value systems, etc.