But then you read the article and it goes to explain how dealing with files is hard and sometimes data corruption and loss occurs (fair enough), but nothing filesystem specific that could explain why ext4 is superior (or special) and had to be chosen.
So it reads kind of like an excuse for Dropbox, only that it isn't.
The argument this article is making is that supporting additional filesystems is hard. The is meant to refute the allegation that it's trivial to add whatever other filesystems the OS supports.
It mentions that dealing with hard disks is hard (which I believe, since they are flaky hardware). But dropbox didn't say "we won't be supporting this kind of hard disk/hard disk controller", but "we won't be supporting these filesystems". Where's the proof that those filesystems have problems that ext4 doesn't have?
That doesn't mean ext4 is problem-free, but it does mean that other filesystems have different problems that are not fixed by mitigations for ext4's quirks.
(Windows has gone to increasing lengths to accomodate and contain badly written drivers, since most bluescreens are caused by drivers. There is now a subsystem to allow the video drivers to crash and entirely restart without bluescreening.)
Last I checked, providing callback function pointers to binary vendor libraries (read/write adapters for FFT come to mind, allowing on-the-fly metric computation or skipping an intermediate storage for FFT convolution) was only possible on Linux, and with statically linking said vendor library into the software (incidentally breaking binary distributability for GPL).