It's not just about capacity (80% is still a lot), it's that degraded batteries lose their ability to deliver high current under load—so acceleration suffers and voltage sags under hard pulls. For grid storage, you're doing slow, steady charge/discharge cycles over hours, so the same battery that can't handle aggressive driving anymore works perfectly fine. Plus, grid storage has virtually unlimited space and no range anxiety, so if you need 25% more packs to hit your capacity target, you just stack them in a warehouse where real estate is cheap.
Also, batteries will degrade faster over time when they start to degrade, because they need more frequent charging. Their internal resistance increase and that promotes heat buildup during fast charging/discharging, another thing that promotes degradation. Slow charge/discharge cycles also help with heat management.
They claim to have taken the Moss Landing fire into account with how they are placing their batteries. We won't know if they've really solved the problem or not until their first battery pack experiences a runaway thermal event.
