I think I understand the concept behind the penalty transaction. What I don't understand from that quote is what other funds there are that "any funds you received after this have been stolen by my partner".

So let's say 2 nodes (A and B) have a channel with 40% / 60%, respectively. Then, a new balance was updated and ended up with 30% / 70%. Supposedly, the 40% / 60% commitment transaction is revoked and no longer valid. However, node B goes offline and node A broadcast the revoked commitment transaction with 40 % / 60% and node B fails to detect this breach because it was offline long enough for the timelock to expire. In this scenario, node B will get only the 60% of the funds that were allocated by the broadcast of the revoked commitment transaction by node A.

The mentioned quote says:

So, what any other funds there are to be received by node B other than the 60%??? This is the part I'm not understanding!