df with APFS: still working?

I don't think this is a question of "reliability", it is simply that the two programs display different things. "Free space" sounds simple, but in reality it is quite complex - different people, different systems, etc. all consider "free space" to be different things.

For example "free space" could mean "how much of the disk space is not currently used for any meaningful data", or it could mean "how much more data can I store here" or something completely different.

The "About this Mac" tries to take the latter approach. If I add more things to this disk, how much can I really add. Technically it involves the calculation taking into account things that are temporary in nature. Often the largest differences here comes from local TimeMachine snapshots (they're created on the source volume, not the backup disk). These are temporary and will be removed automatically.

In contrast, df takes a very "technical" approach and shows the "raw" file system information according to how much of the disk space isn't used for anything meaningful. I.e. even for example temporary TimeMachine snapshots are included in "used" as they're actually storing useful data.

With APFS, it seems that the "slices" aren't really in separate partitions.  Here's what mine shows:

WGroleau@MBP ~ % df -g
Filesystem     1G-blocks Used Available Capacity iused      ifree %iused  Mounted on
/dev/disk1s1s1       931   13       119    11%  563932 9767414228    0%   /
devfs                  0    0         0   100%     662          0  100%   /dev
/dev/disk1s5         931    2       119     2%       4 9767978156    0%   /System/Volumes/VM
/dev/disk1s3         931    0       119     1%     788 9767977372    0%   /System/Volumes/Preboot
/dev/disk1s6         931    0       119     1%      18 9767978142    0%   /System/Volumes/Update
/dev/disk1s2         931  794       119    87% 3003929 9764974231    0%   /System/Volumes/Data
map auto_home          0    0         0   100%       0          0  100%   /System/Volumes/Data/home

Notice that the capacities for disk1 slices add up to 102%. And they all differ. But the total size and amount used is the same for all—and 931 GB is about right for the entire one terabyte SSD.

So, instead of looking at the capacity column, I should compute 119/931=12.8% free (87.2% used).