Mueller wasn't specifically investigating "collusion" with Russia. The first part of his investigation was to determine whether or not the Russians interfered in the election: They did. Hence the indictments. Again, if you read Barr's letter he said there was no coordination between Trump & his campaign people with the Russian GOVERNMENT. It didn't say there was no coordination with Russian mob, Wikileaks, or others. Also, it specifically said it was BEFORE the election- nothing about any coordination AFTER the election. Remember that they opened another investigation to see if Trump was coordinating with Putin/Russia NOW. Barr's letter completely skipped over that part. It didn't mention that the Russians hacked the RNC. It has vast omissions.
Meanwhile the grand jury is still going. Mueller's part as SCO might possibly be over, but the investigations, indictments, & prosecutions are not. Barr may have said there were no sealed indictments from the SCO, but that doesn't mean there aren't more from other agencies. Furthermore, it would be illegal for Barr to even comment/confirm that there are sealed indictments because that is private/classified.