"Das" is not only an article here. It refers to something not mentioned here. It can be substituted by "dies" or "welches". Isn´t it possible to substitute "that" in the English translation? The linguists amongs us can explain it better. I´m just nativ German.
The 's' is not genitiv, it is for neutral nouns.
"Ein schönes Haus." "Gutes Essen" ..."ehrenamtliches Engagement"
Das Beispiel mit Genitiv ist nicht korrekt. Es muss heißen:
Das ist eine Art des ehrenamtlichen Engagements.
This sentence would change the meaning a little. While the fist sentence makes the action to volunteer work, the second version explains volunteer work and makes the action part of it.