But in all fairness it seems like a reasonable system, given the narrow scope of its goals. It does not scale, but that's on purpose. Although I could still see "Feed Aggregation" becoming impractical even with a small number of friends with a modest number of posts.

Cryptographically, a problem is that it makes ciphertexts publicly enumerable, protected by a X25519-derived key. This makes it very vulnerable to harvest-now-decrypt-later attacks, if you believe quantum computing will ever happen.