The function of the chests doesn't perfectly fit the definition of either a cache or a buffer. In my opinion, it's closer to a cache, but that can be debated.
I suggest that a more compelling reason is that people in Factorio communities have been using the phrase "buffer chest" for a while to refer to a setup where a chest is put in between a producer and consumer of resources — common in train stations, heavily-moduled assemblies, and sorting facilities. This conflicting terminology is likely to cause confusion and misinterpretation when these new chests become available. On the contrary, the phrase "cache chest" is very rarely used.
Evidence:
- "Buffer chest" has 63 results on the Factorio forums, 69 results on the subreddit, and 6 results on the Steam forums. The results are overwhelmingly talking about regular, non-robot-connected chests.
- "Cache chest" has 2 results on the Factorio forums, 2 results on the subreddit, and 0 results on the Steam forums. Of the 4 results, 3 are asking for or trying to replicate the new logistics chest.