The Ledger also has two processors and that added very little security. Bad firmware on the insecure chip could completely fool the user, because the secure chip didn't have access to any of the peripherals. Thus the bad chip could ask for a different payload to be signed (or could skip verification entirely) and so on.

Well it added some security, they weren't able to just extract the seed and PIN like they did with the Trezor. Instead of immediately stealing funds they'd have to return the Ledger to the user, and a careful user could check the signed transaction on another device.