The Byzantine "Empire" in 1204 was barely an empire at that point anyhow. It was essentially a rump state consisting of Greece and small portions of Anatolia.

Or, if the Byzantine Empire can be treated as an empire in 1200, the Kingdom of Hungary or the Kingdom of Poland should be treated as an empire as well.

The sack of Constantinople was 30 years into the future after this expedition. But yes, after the fall of Constantinople in 1204, it never was able to recover its former power. Manuel I Komnenos really blew it with the decision to anger the Venetian's.