We're totally taking "free flowing" out of context because of The Bullshit Whisperer's misquote on Twitter.
Free flow describes the old Mike Brown and Heat Iso based offenses where LeBron initiates at the top of the key and people stand around on the free throw line or move around on their own volition but with no real play in mind and waited for LeBron(or Wade in some cases) to either shoot or dish after drawing a second or third defender.
LeBron was saying this new offense, much like the Spurs, is not "free-flow", but rather read and react with lots of counter branches of the tree for each player to do do depending on how the defense reacts to a first set or motion.
Free flowing may sound like a nice term but its not what LeBron trying to describe the Blatt system as at all. This system is tons of motion, counters, picks, screens, spacing, etc. Instead of the coach calling one specific play that will result in one specific shot, a play will get called and there will be tons of different read and react options, predicated my match ups and how the defense responds.