I think where people commonly confuse it is because most often when they see a M.2 device, it is the variant of the slot that exposes PCI-E pins to the NVME SSD

I think where people commonly confuse it is because most often when they see a M.2 device, it is the variant of the slot that exposes PCI-E pins to the NVME SSD, such as for a $100 M.2 2280 SSD from Samsung, Intel, WD, etc. As you mention there's lots of other things which can be electrically connected to a motherboard's I/O buses in a M.2 slot.