Pardon my ignorance, but are the flow batteries you mention the same type used in existing electric vehicles? It can't be assumed that the energy density of flow batteries will ever match as gasoline, but people might be willing to accept the difference if it means more infrastructure adoption and less time spent recharging vehicles.
I've not seen flow batteries in existing vehicles. Electric vehicles use NiMH batteries (0.25 MJ/kg), lithium batteries (0.4 to 0.7 MJ/kg) or lead acid (0.14 MJ/kg fork trucks, but they need mass anyway).
Guessing at the numbers MIT is avoiding naming in that press release, they might get to something three times better than the current lithium batteries, but you also have to add in the mass of the pumps, and the reaction part of the battery.
It could "win", but it won't be a game changer.
Putting aside vehicles though, flow batteries have some interesting properties for utility scale energy storage. The rate at which you can store or retrieve energy is independent from the amount of energy you can store. The first is by the size of your reactor, the second is by the volume of your storage tanks. It is possible if they are using common enough elements for their fluid that they could be a good answer for peak load shifting or production storing.
...we estimate that optimized SSFC systems using established lithium intercalation compounds could have energy densities of 300–500 Wh L−1 (specific energy 130–250 Wh kg−1), which would satisfy metrics considered necessary for widespread adoption of all-electric vehicles.[4] Further improvements would be possible by ‘dropping in’ higher-energy-density or lower-cost storage compounds in the SSFC platform as they are developed.
