Many elements of being the "best choice" are thresholds. Excess performance doesn't make it better. Price is extremely important, but power density is not so important for most use cases. So if it's even slightly cheaper, expect to see a lot of it.
Christmas present for a battery enthusiast? :D
I think the argument against that is that we've spent tens of thousands of person-hours a millions of dollars more researching, developing, improving, and refining lithium-ion batteries than we have flow-batteries, and the purpose of the project in OP is to make it easier to tinker with flow-batteries.
Think of how much better lithium-ion batteries have gotten since we first started using them. We went from "You simply cannot achieve the energy density to propel (tethered) cars without fossil fuels" to "Oh wait we have electric cars now." in like a decade, and in the decade since them, lithium ion batteries have improved in every metric by at least an order of magnitude. We simply don't know how good flow-batteries can get, because we haven't tried, and it's silly to say "why bother trying, they're not very good right now" when we've just seen how much of a difference it makes to invest into this sort of technology.
On top of that, you don't have to match the internal resistance to match power. If you have plenty of material to absorb the heat, then you can tolerate more percentage points of loss.
In particular, while lithium ion batteries can be built to sacrifice discharge rate for a bit of extra capacity, something like a 3C discharge rate is easy enough to reach. And if your use case is powering a building for several hours, you might only need a .2C discharge rate. That would mean lithium ion as a technology is 15x overqualified, and a flow battery that gives you 10x less power would still be overqualified.