It's worth noting that there is fairly extensive video of an unhelmeted officer getting hit in the head with a fire extinguisher thrown during a brawl. I don't know if that officer has been identified as Sicknick or not, but that particular event (an officer getting hit in the head with a fire extinguisher) did happen. Separately, we know Sicknick did physically engage with the rioters, as the official US Capitol Police statement cited in the article you linked said "Officer Brian D. Sicknick passed away due to injuries sustained while on-duty."
So, _a_ fire extinguisher attack did happen (among many other attacks on officers during the riot). An officer did die as a result of injuries sustained during the riot. An anonymous source linked those two events, a link which now appears to have been incorrect. News sites which covered the events in question have effectively released retractions and calls to wait for more evidence.
What more would you want them to do? They can only report on the facts as known at the time, and release updates if those later change. Both of those things were done.
This is an entirely different situation from Fox and OANN alleging massive conspiracies and voter fraud and continuing to hold that position for political reasons despite a lack of evidence.