yes but there is room left and not everything working is listed
a quick glance in the drivers of the pat file from 6..2.3 shows more 10G nic's then just intel
these are the drivers that belong to the list of nic's they support but you can use all alder hardware supported by this driver, the drivers usually have a list of supported pci id's
qed/qede
version=8.33.9.0
description=QLogic FastLinQ 4xxxx
atlantic
version=2.0.5.0+sh
description=aQuantia Corporation(R) Network Driver
-> imho this should enable pretty much all the cheap aqantia based cards like from asus xg-c 100c
ixgbe
version=5.1.3
description=Intel(R) 10GbE PCI Express Linux Network Driver
this is not listed but present too and i have just scrolled through it and took from memory whats a 10g nic driver, there might be one or two more
mlx4_core, mlx5_core
version=4.1-1.0.2
description=Mellanox ConnectX HCA low-level driver
description=Mellanox Connect-IB, ConnectX-4, ConnectX-5 core driver
-> that looks the same as 3617 so i guess connectx-3 will work too (can be checked by pci id's)
bnx2x
version=1.713.36
description=QLogic BCM57710/57711/57711E/57712/57712_MF/57800/57800_MF/57810/57810_MF/57840/57840_MF Driver
i40e
version=2.3.6
description=Intel(R) 40-10 Gigabit Ethernet Connection Network Driver
and beside the drivers in the pat file, as long as its possible to load drivers not signed from synology its possible to compile your own drivers and load them
you would need to copy them to the system and would have to mod the files loading kernel modules (or do a script for loading) but its possible to do that (even if i'm not planning to do that), in contrary to the original 918+ this unit has a pcie slot and also two M.2 slots that beside cache could be used for pcie cards (with adapter M.2 to pcie slot, mainly made for notebooks with a 20-50cdm cable)
it does, when its supporting the newer versions its usually valid for older ones, worst case in some scenarios might be the need to manually copy additional firmware to the system (like its with i915 driver)
Ethernet Controller 10-Gigabit X540-AT2, device 8086:1528
from 1621+ 6.2.3 ixgbe.ko driver: alias=pci:v00008086d00001528
so yes it does support the card
so as a summery i'd say its a nicely usable original unit, the 4x1GBit is questionable (as the 4GB RAM default is) but it can be extended and the default driver set isn't to bad so it will not burn a hole in your wallet when adding a 10G nic, especially mellanox and aquantia are cheaper ones