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The USB-IF document referenced by the article is only the cable specification. The USB Power Delivery 3.1 specification was published yesterday and goes into more detail on the mechanisms.
In brief (from a quick read): a new extended power range is used for >100W to 240W. It supports fixed voltage supplies able to operate at 28V, 36V or 48V as well as adjustable voltage supplies up to those limits. In all cases, maximum current is 5A so the higher fixed voltage options go up to 140W, 180W and 240W respectively. There is an additional negotiation phase (between source, sink and cable) before stepping up from standard to extended power range.
According to everymac, 2015 iMacs use 5th (21.5") or 6th (27") gen CPUs, which do have Quick Sync:
Later processor generations improved Quick Sync and added more video formats. 5th gen (Broadwell) added some VP8 support. 6th gen (Skylake) added 8-bit HEVC encode/decode. The 2015 21.5-inch iMac (5th gen) is excluded from the list of supported models, but the 2015 27-inch iMac (6th gen) is included so Quick Sync in Broadwell does not suffice, implying 8-bit HEVC encode/decode is the required feature.
The listed models have one feature in common: all have a Skylake or newer processor.
Skylake adds hardware encode/decode support for HEVC, so this might just be a case of the Sidecar feature being implemented with HEVC rather than H.264 to reduce bandwidth requirements or allow a higher frame rate. Older Mac models would have to do HEVC encode in software, so enabling the workaround might impose a significant CPU performance load when the display is rapidly changing.
If I'm right, this could impose a similar requirement on the receiving iPad: basic HEVC hardware decode is a feature of the A9 and later processors, so A8 models that can run iOS 13 (iPad Air 2 and iPad mini 4) may be unable to act as a sidecar receiver as they would have to decode the video stream in software.
There was a similar cutoff with the introduction of AirPlay video support: it required a Sandy Bridge or later processor to get H.264 hardware encode support. Third party software (AirParrot) implemented that feature on older Macs by doing the H.264 encode in software, but it caused a fair amount of CPU overhead.
AppleInsider said:Apple has also confirmed macOS Mojave will be the last version of the operating system to support 32-bit apps "without compromises."
The next slide said "Mojave is the last macOS release to support 32-bit apps".
"Without compromises" is in reference to High Sierra, exactly what Apple was saying last year. Mojave will run 32-bit applications, but with compromises. Apple has not said exactly what those compromises are.
The version after Mojave (presumably 10.15, due to be released in late 2019) will not run 32-bit applications at all.
More points of suspicion: "MacBookPro14,3" is the model identifier of the existing mid 2017 15-inch model. I doubt a new model with DDR4 memory and a newer CPU architecture would have the same model identifier. If it is using DDR4 it must be desktop memory, since the Core i7-8750H doesn't support LPDDR4. That suggests a new motherboard and structural changes to cope with more heat produced by the memory. The motherboard identifier is identical to other MacBookPro14,3 listings on Geekbench (apart from a "1.0" suffix instead of "MacBookPro14,3").