Rashi (citing Shocher Tov): This teaches that Bnei Korach spoke for these generations after them. Had they spoken for themselves, they would not say "Avoseinu Sipru Lanu." They themselves saw the miracles in the Midbar, at the sea, the Yarden and Yehoshua's war.

Malbim: Something that one received via oral tradition, it can be false, for two reasons. (a) The first teller made up the tale. (b) An intermediate teller made up [matters] on his own, or he did not understand the matters properly. However, our tradition of the early miracles is Emes from every side. (a) they did not receive the matters from other tellers - we heard with our ears! (b) You cannot say that those who received the tradition fabricated it - our fathers told us. Fathers do not bequeath false Emunos to their children! A tradition like this from fathers to children is true.

Malbim: You did then a new action not dependent on natural orders, which is called Ma'aseh. Pe'ulah is via miracles above natural order.

Radak: This refers to the earlier Avos. These miracles were not in the days of the Avos of the people in Galus. Rather, it is the Avos who entered the land. They saw awesome miracles - they conquered the land without war. They told their children generation after generation, until Bnei Galus.

Malbim: This was in their days, long earlier; they saw with their eyes. Our fathers did not see.