I decided to risk it and got the G7 7588 and a Samsung 970 EVO 2TB M.2 ssd and it works just fine, Windows is reporting capacity of 1,999,873,593,344 bytes (1.81 TB). So +1 to the Dell engineers, -1 to their manual writers.
The specs in documentation are often based on what the system is actually available to be ordered with or the largest capacity SSD they actually tested. If the manual had said, "We don't know of any practical limit on supported SSD capacity" and then somebody tried a larger SSD later and found it didn't work, people would be upset about that too. And if the manual said, "We tested up to X capacity. As far as we know, higher capacities should work, but we can't say for sure, so if you want to try it, you're on your own and good luck," that would be a bit awkward.
In any case, there is currently no hardware-level practical limit on supported storage capacities on either SATA or NVMe storage. The last time capacity had hardware/firmware-level limitations was in the Windows XP days where you needed SP1 and updated motherboard firmware to go beyond I think 137GB, because that required support for 48-bit LBA addressing. Then there are some limitations around using disks larger than 2TB as boot disks if you're using Legacy BIOS booting, but most systems these days are using UEFI, which doesn't have that limitation. Other than that, the only limitations are around what the drive manufacturers can produce and what consumers can pay for.
That said, there are some threads here reporting issues specifically with the 970 Evo on some systems, even when the 960 Evo worked fine. I haven't seen any fixes for those reports or even an underlying cause identified, but I would have called that out if I'd seen your first post before you pulled the trigger. I think it's been mainly XPS systems though, and in those cases the disk isn't even usable, so obviously that doesn't apply to you. I hadn't seen anything either way about the 970 Evo on the G5 and G7 laptops until this.