Floppy-disk based systems are not the only way to ensure security/reliability, but it's far better than a half-baked system that isn't properly audited. Systems we rely on must be dead simple and reliable.

There was no alternative to floppy disks when they were built though. Was there a deliberate choice to use an older technology?

I'd assume that's more about not fixing something that isn't broken. (Because if you break it, you might accidentally annihilate humanity!)

Do you have a source on this?

Google it, it seems to be a generally known fact: https://www.cnn.com/2016/05/26/us/pentagon-floppy-disks-nucl...

This is when I head off to n-gate.com