2^64 nanoseconds is > 580 years. Even cutting it in half for signed values is still likely 10X the expected lifetime of this particular iteration of APFS.
The granularity is probably useful for establishing an ordering (even somewhat arbitrarily) for highly concurrent file system operations, and thus potentially skipping a bunch of expensive synchronization.
Found this: https://github.com/jbenet/nanotime. No idea if related in any way or used for APFS but it was interesting enough for a quick read. This one is MIT licensed. A little more searching I found a variant but it's done with big-endian so it's much different.
Why would this go into a new standard? Is there a practical use for such a high level of granularity?
edit: off by a factor of 10 in my calculation... It's actually
2554-07-21 23:34:33.709551615 UTC