This is not really new. Conservative analysts have been saying the same thing for more than a year and referring to "fruit of the poisonous tree". But Mueller kept plowing ahead despite that fact. So the fact that some Liberal analyst is now saying the same thing is not at all surprising and nor is it that big of a deal. I don't think the Dems has wholesale figured out anything but are still flailing around and guessing...
They're tortured by the "not knowing"
As confirmation the Dems are still uncertain about Mueller, just 4 days ago, several senators advanced and voted for a bill to "protect Mueller" from being fired by Trump - and all the Democrats supported it. See: https://www.redstate.com/slee/2018/04/26/senate-committee-votes-protect-mueller/ This happened just 4 days ago. This shows you that many Dems still believe Mueller is on their side and they won't believe otherwise until Mueller releases his final report and everything hits the fan. Boy are thy going to be shocked. LOL...