There's a difference between "a foreigner" and "a stranger". A foreigner is a person who comes from another country. A stranger is an unknown person. Someone you know nothing about. So, when you go to a country which is not yours, you'll be considered as a foreigner (as you come from another country) and also as a stranger (as nobody knows you there). Suppose you stay there for a long time and people start knowing who you are, you'll be no more a stranger but still remain a foreigner.

The word "stranger" doesn't imply you're from another country. It just means you're someone the other person doesn't know. Children are told not to talk to strangers. It's similar to "cizí člověk" (stranger) vs "cizinec" (foreigner). So it was a bit like saying "come on, guys, you don't even know me".

Precisely speaking, you are also a stranger in this case. ／Generally speaking, however, a foreigner is not necessarily a stranger. For example, you would never call a foreigner "stranger" if he/she were living there for many years and very well known by all the people living together in the community.

If someone were a stranger, he/she could be from Ireland and even vote in the election referred on the leaflet. On the contrary, a foreigner could not vote in the country where he/she is staying now. That's probably why they said "You are not a stranger, but a foreigner.".

I suppose that "foreigner" means "someone who is from another country" while "stranger" just means "someone who has come to the place from another place and doesn't know about the place and the people living there well".