I think you may be confusing the minimum standards for copyright eligibility
with the FSF's recommendation for when to use a copyleft license. They
recommend using a copyleft license when a program is over 300 lines of code,
and licensing anything with less than 300 lines of code under a permissive
free software license.
For files included in a software distribution that is part of the GNU
project, the FSF requires licenses on all files greater that 10 lines of
code, because that is approximately the "minimal creativity" required for a
program to be eligible for copyright.
By the way, I'd recommend using the CC0 license instead of just putting the
text "public domain" in your code because it is more legally
watertight[1][2].
https://creativecommons.org/choose/zero/
[1] https://www.gnu.org/licenses/license-list.html#PublicDomain
[2] https://www.gnu.org/licenses/license-list.html#informal
Copyright licensing does get confusing. It would be easier not to not have to
worry about it.
