The cache policy of write-back or write-through applies only to cache-able accesses. For device or strongly ordered, the data must be written without entering the cache. For a device access, the merging store buffer can be used to temporarily hold the data while the write to the device completes. The CPU can go and do other things.
As per Wikipedia's write buffer, a write-through cache does not need the eviction buffer. The merging store buffer is not really a cache feature and could be looked at as a completely different entity. But it has to do with memory access optimization (like a cache) and is in the same functional block.