But they said "everyone" - I imagine undervolting would only be done by people trying to hack the sgx, or maybe try to save a bit of power. So I'd assume 99.99% of users would be unaffected? Or did the cpu undervolt automatically before the update?

FWIW I have undervolted every laptop I've ever had. It's a huge win in terms of battery life and often performance as well because you can avoid ever hitting the thermal envelope. It can be a 30% boost if you have good silicon.