To be fair they would have needed to spend time testing the manual implementation as well.
The problem I see mainly is that even if this rolls out perfectly, the erratic and changing nature if llms still make it pointless as a proof of concept. Next time Claude might fuck up in a fringe way that's not covered by unit tests and is missed by manual tests.
On the other hand I guess I've been guilty myself on numerous occasions to implement fringe bugs into production code, but at least I learn from it.