The sourcecode wasn't leaked - an internal website (with the software, written in C#) [1] was leaked, and then the program was decompiled and pushed to GH.
EDIT: The website leak itself is actually pretty old news, but the decompilation and public shaming of the code itself is relatively new. “Zaufana Trzecia Strona” has more information [2] about the leak itself (in Polish).
Aren't you talking about backend? The code on github is a client used by a polling place and it was publicly available (this is how they've it distributed)
EDIT: The website leak itself is actually pretty old news, but the decompilation and public shaming of the code itself is relatively new. “Zaufana Trzecia Strona” has more information [2] about the leak itself (in Polish).
[1] - http://zapasdlakbw.home.pl/kalkulator-wyborczy/kalkulator/
[2] - http://zaufanatrzeciastrona.pl/post/wersja-testowa-systemu-p...