Does APFS reduce the needed free space on SSD's?
If we assume that the Anandtech study's conclusions are correct, then no, there's no change in the needed free space when using APFS instead of HFS+. You would need no more and no less free space.

It is important to note that the "free space" that Anandtech talks about is not the free space inside the file file system (i.e. part of the filesystem structure that is not currently used for storing user data), but rather free space outside the file system (i.e. part of the disk that has never been written to).

As both HFS+ and APFS support the TRIM command, the essential here happens outside the file system, and thus the change from HFS+ to APFS does not affect the result.

However, you might still observe a performance difference when using APFS compared to HFS+. This is not caused by the free space issue itself, but rather that APFS and HFS+ simply have different performance characterics. Some things APFS does faster than HFS+, and vice versa.