It's a Health Plan. People go out of network in Kaiser all the time- basically, once Kaiser's default system can't handle you (complex organ transplants, advanced rare cancers, etc), they send you to a place like UCSF, and have negotiated the cost structure.

There's no guarantee KP won't bankrupt you, but it seems better than most.

If you voluntarily go out of network for non-emergency care, then yes, you pay a ton. If you have an emergency and go to an out of network ER, that is still covered (at least for plans purchased on CoveredCA) because of provisions in the Affordable Care Act. The billing may be a logistical challenge, but it should not bankrupt you!

Kaiser has both HMO and PPO plans, although depending on your employer you might only have access to the HMO (e.g. if they went with another insurer for the PPO).

Thus proving the inherent value of the network!