It seems very much like the B+ tree approach is just a mental model put on top of the exact same idea that is being argued against. The initial list of "bad things about LSM approaches" has almost exactly the same items on it as the list of features the B+ approach claims to achieve.

Maybe I'm getting this all wrong, but aren't the leaves also representing chunked data, which is compressed.

The Prometheus solution also sequentially places compressed chunks for the same series. The time slicing actually has a lot of benefits and can simply be seen as the first level of the described B+ tree. An index of chunks for a series can then be seen as the second level.

The potential read amplification here seems completely equivalent. Just from my high-level view, all properties of the read and write path seem almost identical.