"Only those from the northern slopes (in present day Poland) called themselves Lemkos"
Not true - there were/are lots of lemko villages in slovkia.
Not true.
Carpatho-Rusyns from Slovakia (like my paternal grandparents) never use the term "Lemko" to refer to themselves. They use it to refer to Carpatho Rusyns across the border in Poland (i.e. my maternal grandparents).
It's really just semantic, they are ethnically nearly identical, but the terminology was not.