no, you can totally run the m.2 at 32Gb/s as long as nothing else is competing for bandwidth. PCIe 3.0 x4 = 32Gb/s.
Adding something else, e.g. a second fast m.2 such as the new samsung ones, you’ve suddenly only got 16Gb/s for each m.2 (and that is assuming nothing else is competing for bandwidth)
Think about it this way. You’ve got a server connected to your network at one gigabit. You’ve got two computers connected to your local network, also at one gigabit each. Three computers on your network, all connected at 1 gigabit. If both of those computers are pulling data from your server computer, do you expect both computers to both be pulling 1 gigabit of traffic from the server each, for a total of 2 gigabit? No, they’re both competing for your server’s 1 gigabit connection, each only gets half, 500 megabit each or a total of 1 gigabit.
Now if you only use one m.2 at a time, you can get 32Gb/s out of each of them, just not at the same time. Just like the ethernet analogy 
Bad news for anyone that wants raid 0/1 for speed, though, in m.2 land.
The 24 DMI lanes is 24 input channels, but upstream to the cpu, only 4 channels of bandwidth. The implicit assumption is that most of the peripherals attaching to the DMI don’t need the full PCIe bandwidth. That’s mostly not wrong, except in the case of m.2.
the m.2s are connected with everything else that goes through the PCH. i211 ethernet goes through the chipset, but like the i217/218/219v doesn’t use any PCIe resources. The ethernet card has direct resources on the CPU that doesn’t use lanes.
this is something intel did to encourage OEMs not to put non-intel nics on their boards to save a few $$. The DMI/chipset based ethernet adapters back on Z87 and before had a real hard time because that DMI interface was slow, not pci-e like and had a lot of implementation headache. the DMI of that day was not really designed for peripherals even as fast as gigabit ethernet (not in terms of bandwidth per se, but in terms of interrupts per second and some other params like that). DMI3.0 fortunately is a huge improvement over the stuff we saw on Z87/sandybridge/etc.
The narrow execption for the one specific intel ethernet adapter aside, All m.2 plus downstream USB (asmedia especially), sata, plus any other PCIe peripherals onboard like the sound card, all through the PCH lanes, through that single DMI 3.0 (PCIe x4 equiv) link to the PCH
But with a x8 / x4/ x4 config, I assume they all run to the CPU.