Although there are, as you might expect, Albanian speakers in the area of Greece adjacent to Albania, the Albanian speakers in Epirus are known as 'Arvanites'. Arvanites are part of a group that migrated into Epirus a long time ago, some time in the middle ages. The cause of their migration isn't well known, but for large periods of time Greece and Albania were part of the same large Empire (first the Byzantine Empire, and then the Ottoman) and Greece was a regional economic centre while Albania wasn't, so it seems reasonable to guess that at least part of their motives were economic.

Arvanites, unlike many other Albanians, are uniformly Orthodox Christians (unlike many of the other Albanian minorities, who are mostly Muslims), which may be partly why they have historically integrated very well into the larger Greek community. Arvanites generally fought alongside Greeks during the wars of independence, and this has generally led to their being slowly integrated into the Greek state, first becoming bilingual, and then slowly losing their native language. There was never any purge or anything like that, they've just been absorbed. I expect there are many people in Greece with Arvanite descent nowadays, but the language has been largely lost.

As an aside I think Arvanite, while closely related to Albanian to the point of being mutually intelligible, is generally considered a different language, although I expect the mapmaker wasn't aware of this distinction. (It doesn't help that people in Epirus at the time probably would have referred to their Arvanite neighbours as 'Albanians' and their language as 'Albanian').

Source: Aristides Kollias, 'Arvanites and the origins of the Greeks'.