There's a difference between smart leftist criticism and dumb leftist criticism.
We saw in the 2016 primary that Clinton moving left was received with indifference at best and outright scorn and derision by certain factions of the left. Most of the people dumping on Harris and Booker now aren't going to be convinced by this move, they'll just focus on something else or ask why they didn't support it to begin with or some other mental leap.
Like I don't particularly trust Booker to follow through on progressive policy outside of some red meat for the base, but he also doesn't need to do much to win my vote if he's the nominee.
I am tuned in to a lot of lefties and Clinton's shift was not unnoticed. It was in fact celebrated as a win for Sanders' run.
To expect that to immediately have switched people to be 100% on the Clinton train is silly.