I'm not sure why this detail is relevant. The CPU it consumes is still CPU. Hypothetically, if a rewrite saves $100 million annually in compute, why does it matter that the majority of its time isn't spent in compute? It's still $100 million.
> then saving that tiny bit of CPU actually saved no money
this doesn't follow
> since it was already riding on the spare capacity of other investments
nor does this
Take for example a CLI that downloads and verifies the Bitcoin blockchain. It may spend most of its time downloading blocks, but it spends a ton of time calling SHA256 to verify those blocks. Similarly with a tool that downloads and checksums large files like Docker images.
If you have a fleet of 650K developer machines all running this util, then at some point it becomes cost effective to optimize the CPU usage.
Whether that point was reached in this example is not something I know. It seems like the L7 and their manager believed it was. But OP believes it wasn't. Either way, we don't know from OP's description of the situation.
Rewrites to save compute resources are done pretty frequently at huge companies. It's a relatively easy source of projects for high level engineers because they can see the fleet-wide metrics and have figured out how to choose projects that have clear dollar value in their performance write-ups.
Could an L7 have wasted a bunch of time on a fruitless project? Of course. Could an L7 have wasted a bunch of time on a fruitless project at a notoriously cutthroat and frugal company AND be super proud of it? That seems much less likely since they have to show some numeric impact at that level, but I obvious still possible. Would their manager then also be proud of it? That seems even less likely.
So when the only description of the project also accurately describes a large class of valuable projects (i.e. decrease fleet-wide cost of a ubiquitous tool) I was genuinely curious if there was more to the story.