Maybe this will clarify (letter to talkingpointsmemo.com's publisher and main editor Josh Marshall):
https://talkingpointsmemo.com/edblog/the-barr-gambit
I'll summarize:
1. The letter is from a former federal prosecutor.
2. A quote (from the former prosecutor): "A few thoughts on the Barr Gambit, which I think will go down as a singular achievement in the annals of intellectual dishonesty and bad faith legal jujitsu:"
3. The letter writer points out that there is no real dispute that the Russian government "brazenly interfered" in the 2016 election _in support_ of Trump. Committing several crimes (hacking, conspiracy to defraud among them). The intended beneficiaries of the criminal behavior was Putin/Russian governemt and Trump.
4. Trump and his campaign "joyfully used and weaponized" the stolen information re: Hillary.
5. Mueller's team did not find enough evidence to charge Trump criminally (at least apparently), but there is no question Trump/Putin were the biggest beneficiaries of the crimes.
6. It appears to the letter writer that Mueller had significant evidence of obstruction. Barr relied apparently in part on Mueller's conclusion he could not bring charges of conspiracy/whatever to argue that Barr must therefore indicate the lack of Trump's corrupt intent to obstruct.
7. This is legally wrong (obstruction does not depend on the existence of an underlying crime - just the fact of an investigation or proceeding is occurring. He/she gives an example: if you receive and use stolen money, even if not involved in the theft, you have interest in the thwarting of an investigation to continue to keep and not forfeit the ill-gotten money. The Mueller investigation was a direct threat to the Presidency, as well as a threat to investigations of his "shady business empire." The argument Barr uses might hold up if the election interference were intended to help Hillary and Trump's campaign were not under investigation.
8. There had to be evidence on failure of evidence of conspiracy or coordination - you would not be able to get 500 search warrants, for example. The failure was to reach the threshold of proving a case beyond a reasonable doubt.
9. Declining to prosecute for obstruction (by Mueller, not Barr) could have been because: a) you don't indict a sitting President, you defer to executive authority, or you defer to the legislative prerogative of impeachment.
10. The former prosecutor expects if the actual Mueller report to contain a devastating case against Trump for obstruction of justice, and expects Barr, the White House, and Republicans in Congress to "fight like hell to keep as much of the report as possible away from the public and House Judiary. Democrats cannot let this go."