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.

I don't think chiplets require or even are even generally seen with silicon backplanes yet, otherwise you'd be leaving out stuff like AMD's CPUs.

And from there, stuff like the Vac 9000's packaging were very advanced for the time designs that I'd hesitate to call PCBs unless we're calling all organic packaging PCBs now.

Sort of - there's some distinction in the "multi-chip" vs "multi-chiplet". To some degree though, it's true in the sense of: "A rose by any other name would still smell as sweet".

The explanation by 'kurthr provides some insight as to why we don't use the term MCM for them today, because the term would imply that they're not integrated as tightly as they are today.

The best example of MCM's are probably Intel’s Pentium D and Core 2 Quad. In these older "MCM" designs, generally the multiple chips were mostly fully working chips - they each had their own last-level (e.g. L3) cache (LLC). They also happened to be manufactured on the same lithography node. When a core on Die A needed data that a core on Die B was working on, Die B had to send the data off the CPU entirely, down the motherboard's Front Side Bus (FSB), into the system RAM, and then Die A had to retrieve it from the RAM.

IBM POWER4 and POWER5 MCM's did share L3 cache though.

So parent was 'wrong' that "chiplets" were ever called MCM's. But right that "chips designed with multiple chiplet-looking-things" did used to be called "MCM's".

Today's 'chiplets' term implies that the pieces aren't fully-functioning by themselves, they're more like individual "organs". Functionality like I/O, memory controllers, and LLC are split off and manufactured on separate wafers/nodes. In the case of memory controllers that might be a bit confusing because back in the days of MCM's these were not in the same silicon, rather a separate chip entirely on the motherboard, but I digress.

Also MCM's lacked the kind of high-bandwidth, low-latency fabric for CPU's to communicate more directly with each other. For the Pentiums, that was organic substrates (the usual green PCB material) and routing copper traces between the dies. For the IBM's, that was an advanced ceramic-glass substrate, which had much higher bandwidth than PCB traces but still required a lot of space to route all the copper traces (latency taking a hit) and generated a lot of heat. Today we use silicon for those interconnects, which gives exemplary bandwidth+latency+heat performance.

Putting aside the terminology, a lot of people would have you believe that chiplets started in 2017 but they existed for 20-30 years before that.

Sorry I flubbed that -- I meant what are now called chiplets interconnected and packaged together used to be called MCMs. A chip was always a single piece of silicon (aka die), so chiplets used to just be called chips. There was never any rule that chips in an MCM were "standalone" or functional by themselves like some seem to be saying, in fact earlier computers used multiple chips for subsystems of a single CPU (https://en.wikipedia.org/wiki/POWER2 this thing had individual chips for IFU, LSU, ALU, FPU, and D$). There was never any rule that MCMs were not low latency or high bandwidth or must have a particular type of interconnect or packaging substrate.

Advances in technology and changing economics always shifts things around so maybe chiplets are viable for different things or will make sense for smaller production runs etc., but that doesn't make them fundamentally different that would make them not classified as an MCM like the article seems to suggest. It literally is just the same thing as it always was, multiple chips packaged up together with something that is not a standard PCB but is generally more specialized and higher performing.