The highest military council in the country uses Signal to communicate. I think violating FOIA is probably part of the appeal. Or they use that modified Israeli client that stores messages to address those concerns.
They don't use Signal. They use an app that wraps around Signal. There is in fact a difference. Specifically because the purpose of that app is to do exactly what you're accusing Signal of doing. If Signal already did this... why would they pay for the other app?