HSV secured their top-flight status by hanging on for a 1-1 draw, which saw them win the tie on away goals and maintain their record of being the only Bundesliga side never to be relegated.
"That was the closest of all possible outcomes," said Slomka after the game.
"In the first half we were very, very well prepared. In the second half, the frustration came after [their equaliser]. "
Furth were trying to achieve instant promotion back to the Bundesliga after their relegation in 2012-13.