The Bill Broker computes SPOT FMV as:
spot_exchange_rate * ampl_target
To get the AMPL target price, it reads the same CPI oracle that the Ampleforth protocol uses for rebases. So, while an oracle is involved there is still no reliance on realtime market prices.
However, since the CPI oracle and BB are both owned by the DAO governance timelock, any changes typically take 1-3 weeks to enact. Setting the CPI oracle delay to 28 days seems like a good way to further derisk disruption with minimal side effects.
This change would also have helped in the case last Fall when the BEA revised its historical data. So even outside of the immediate Bill Broker motivations, I think there is benefit to the broader AMPL protocol.