They patched a low-level vulnerability in their boot process. Their phones' debug features would allow attackers to load an old, unpatched version of their (signed) software and exploit it if they didn't do some kind of downgrade prevention.

Using eFuses is a popular way of implementing downgrade prevention, but also for permanently disabling debug flags/interfaces in production hardware.

Some vendors (AMD) also use eFuses to permanently bond a CPU to a specific motherboard (think EPYC chips for certain enterprise vendors).