But it is a cache. Upstream even called their repo a _build cache_ until very recently. The new name of substituter is objectively worse then build cache.

And the cache is on disk and be haves like a cache. There is even min-free and max-free nix.conf variables to trigger a cache GC when it gets too big and stop after some threshold.

Digging into why this cache is safe or functional is too low level for the target audience of the article. A key to writing is knowing your audience.