As I earlier proved the Siberian admixture among Baltic Finns didn't come from East with them, it was already in Finland in the time when Baltic Finnic people reached Fennoscandinavia. My statistics showed that rare alleles being found from Russia and Asia are in Finland just at the same level as in other European countries.
Looking closely the Asian admixture in Russia we can stretch the rare allele source to the Altay region. How did Altaian admixture can be found in Mordvins? Was it brought by Scythians or Mongols? I don't know, but the fact is that it is there.
Scythian sphere according Wikipedia
For adjacent information about Mordva/Moksha see the supplementary figure 11.