The article describes the steps necessary to get the short and relevant answer: how much free space is there left on my physically attached disk devices? On Linux, the answer you're actually looking for is hidden among half a dozen tmpfs/devtmpfs/udevfs/whateverfs "synthetic filesystem" mounts; the situation is even more dire when you type "mount" which will list another couple dozen, with the relevant information (such as, which external media is mounted?) hidden in that noise.
The BSDs, rather than drowning the user in noise, and creating the problem that the article tries to address, provide just the relevant information.
On FreeBSD `du` also doesn't necessarily go e the right info due to us, where having multiple filesystems on the same pool is easy and simple. This leads to many filesystems reporting same free space. Right way there is to check the zpool directly ...
That's 100% a ZFS problem, not a FreeBSD problem ;) I have the same issue on my Ubuntu and NixOS systems. Well that's the price you have to pay for ZFS, it doesn't exactly fit in a traditional model - but the space reported by "du" is still accurate e.g. wrt the remaining quota.
