The silicon backplanes are also soldered on (advanced) PCB backplanes, so they are just interposers.

I do not think that using various kinds of silicon bridges warrants a change in name. It should be fine to call any such product that uses multiple chips as a MCM (multi-chip module). Even for consumer CPUs, MCMs have already a long history, e.g. with AMD Bulldozer variants, or with the earlier Pentium and Athlon that had the L2 cache memory on a separate chip.

The word "chiplet" is relatively new and its appearance is justified for naming a chip whose purpose is to be a part of a MCM, instead of being packaged separately. A modern chiplet should be designed specifically for this purpose, because its I/O buffers need very different characteristics for driving the internal interfaces of the MCM, in comparison with driving interfaces on a normal PCB, i.e. they can be smaller and faster.