Yes, but presumably the authors are suggesting broader application than just caching a system prompt.

The paper's approach should work well if (a) you can calculate KV(A || B) as a function of KV(A) and KV(B) independently, (b) you can identify which documents A1, A2, A3, ... are used commonly enough to be worth caching, and (c) it is cheaper to buy and sell KV(A) on a market than to compute KV(A) when it is needed. Given the size of KV(A) I am not sure that (c) will become true even if people solve the open research problem represented by (a) and accept the state-of-the-art trade-offs known for (b).