The Spark always had 5 native ISOs. All ISO values (400, 640, 800, 1250 and 1600) are equally native, meaning they are all derived from distinct analog gain paths at the sensor level. The global shutter sensor developed for the Spark has scientific/industrial DNA in its roots - multiple independent ADC signal amplifiers are not uncommon for such specialized sensors. We chose to initially feature the “dual-ISO” as this is something people are familiar with, and it’s not technically incorrect - if you limit yourself to using only 400/1600 you will get the expected dual-ISO behavior.
What I am trying to convey is that the benefits, or as you put it “what makes a dual-base sensor special” are the same here. You can treat the 400 and 1600 ISOs the same way you would with the Venice 800 and 3200 bases (only lower), except in this case you have access to additional in-between values. Each of those native ISO values allow you to get the most out of the sensor in finer-grained steps - there are no digital gain applied at any point.
We have yet to run strict analysis & tests to determine the exact distribution of above/below stops for each ISO setting, and I’m looking forward to get those going so we can share the results.
TL/DR: You do have dual-ISO with the Spark, and then some. “Quintuple-ISO” sounds a bit off, but that is what the sensor affords you. Each ISO value roughly retains the full dynamic range, and just like any dual-ISO base as signal is amplified the noise floor gets amplified too.