The calculation is a bit different: 100,000 flashes / (24 x 60 minutes) = 69 days.
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It would be quite strange for Siglent to flash data every minute, so I hope it was just poorly phrased in the firmware notes.
However, the only reason to save data every minute would be for power-down purposes, which must be done in non-volatile memory, such as flash.
I recently photographed a flash memory chip in an SDS5000X. This flash was located near the processor. You can assume that this type of flash is used in all Siglent devices.