The midpoint voltage of the voltage divider used for the reference is 0.45V, well within the -2.5 to +2.5 swing.
The reason why I think it matters is that batteries are a nuisance, they run empty and will cause a problem with long running experiments, so it's worth the extra time to engineer them out of the circuit.
From what I can see they're only used to power the op-amp.
The 'floating ground' trick is s.o.p. when designing op-amp circuitry that needs to be fed from a single supply.
