But length() is standard POSIX, no? Even length(array) has been approved by POSIX [1] but not yet included in the spec (they're very slow to update the spec for some reason). Both forms have been supported in onetrueawk, Gawk, mawk, and Busybox awk for a long time.
[1] https://www.austingroupbugs.net/view.php?id=1566