I assume you're talking about the tweet discussed in this story, during the primaries leading to the 2020 election:
The tweets claimed a police officer “murdered” Michael Brown. But that’s not what investigators, including those from the Obama administration, concluded.
www.vox.com
First of all, unlike the Trump movement, the left generally is not a monolith; there is wider diversity of opinion and less of an attempt to force adherence to the party leader's narrative. So, for example, you did not have a flood of other democrats feeling compelled to echo and support the statements of Kamala and Elizabeth Warren, and in fact some people criticized them. That is an article from a pretty leftist publication, Vox, calling Harris and Warren out for being wrong about the facts.
The second question goes to the issue of what "lie" is occurring, which is really just calling it a "murder" instead of a "killing" or "shooting" with the distinction being a question of intent and belief (something that only the shooter, Wilson, can truly know, but that has to be imperfectly determined by the judicial system). Certainly many of Brown's family and friends still believe that he was murdered. Those statements are certainly contrary to what was determined by the state and federal investigations of the incident. But are they lying? To paraphrase an old Seinfeld bit, it's not a lie if you believe it. But in any event, there's no question that Darren Wilson shot Michael Brown; the only dispute is to his intent and reasonable belief when he did it.
That, to me, is worlds different than what we have regularly seen from the right and Trump for the last decade-plus. For example, compare Harris's tweet and the Brown case to the stuff a bunch of right-wingers (like Sean Hannity, Roger Stone, Newt Gingrich, etc) said about the murder of Seth Rich. Just straight-up making stuff up - or repeating stuff that was made up by Russian intelligence - to push a political narrative. Or take the 2020 election conspiracies that Trump continues to repeat to this day, despite the fact that everything he says has been repeatedly debunked, a lot of it being straight-up baseless lies invented from whole cloth by his advisors, like Sidney Powell and Rudy Giuliani, who have been jailed or faced significant civil penalties for outright lies. Or take the birther conspiracy that Trump literally built his brand on, which again was based on complete lies, that Trump continued to repeat long after they had been continually debunked.
As for the statistic about people on the left estimating the number of unarmed black people killed by police in the thousands, I'd like to see the actual data you're referring to but it isn't surprising; in addition to the issue of people believing what they want to believe, across party lines, there is the fact that Americans in general (probably most people, honestly) really struggle with things like probability and estimating. You can see that phenomenon discussed here:
Americans Have One Very Strange Cognitive Bias. Americans simply have a really hard time assessing the frequency that things happen, tending to overestimate things that happen rarely (like people being transgender or, say, unarmed black people getting shot) and underestimate things that happen frequently.
You seem to essentially be creating an impossible standard here, in which no one on the left is allowed to criticize the right's rampant, constant, pernicious lying unless every politician on the left is perfect too. Again, it's exactly this sort of cynicism and misleading bo-siding that has allowed Trump to flourish in the first place.