Its a mystery..I have seen many Knanya nasrani Christians boasting about their Jewish Syria orgin.. they don't look like a typical Syrian but exactly like that of other Malayali population...look,They never marry outside groups,not even to other nasrani Christian sects to keep their so called blood purity..
But not all nepalis are mongloids...Its a general misconception....
That is why still have that doubt.Climate played a role in here.Like you said nasranis dont marry from outside but now they have malayali features.