...for a particular definition of "PCIe slot".
More accurately, that's a M.2 SSD that will work in an M.2 slot provided g=the slot supports the NVMe PCIe 3.0 x 4 protocol (check the small print in your motherboard manual: M.2. slots are tiny expansion slots specifically for SSDs that can support some or all of SATA, USB3 and PCIe. https://en.wikipedia.org/wiki/M.2). By contrast, here's a SSD that does plug into what people usually mean when they say "PCIe slot": http://www8.hp.com/us/en/workstations/z-turbo-drive.html - actually, looks like it's actually a "Standard PCIe" to M.2 adapter with a M.2 card on it, so it nicely shows the difference between M.2 and a "PCIe slot". (NB: the Intel platform controller chips now allow certain PCIe lanes to be specifically optimised for SSDs, so SSDs plugged into "standard" PCIe bus slots might lose a slight edge over the ones in dedicated SSD slots...)
Now, the PCIe protocol in "PCIe slots" is also used by stuff soldered to the PC motherboard, as well as by ExpressCards, Thunderbolt some M.2 slots and, yes, Apple's proprietary SSD interfaces. ...so, if you want your M.2 card to be "a PCIe SSD" because it uses the PCIe interface (reasonable), then I'm afraid that the Apple card is "PCIe" by the same argument.
The important distinction performance wise (which was the origin of the argument) is that Mac SSDs use PCIe and not SATA - and the latest iMacs, at least, use PCIe 3.0 x4 NVMe SSDs which are in the same league as your M.2 drive - I think Apple may even have gone PCIe before M.2 PCIe options were so widely available.
However, you're quite right that the Apple world would be a nicer place if they'd use M.2 slots instead of proprietary ones. Plus, the nMP SSD is only PCIe 2.0 so it will be a bit slower.
But a SSD that talks to the processor via PCIe is still a PCIe SSD, even if it requires Tim Cook to personally stand by your Mac and hold the wire in place.
) The real slow boat from China! 
