POSIX allows for calling the length function w/out any parentheses:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html

length[([s])]
Return the length, in characters, of its argument taken as a string, or of the whole record, $0, if there is no argument.

simple testcase:
$ echo asdf | gawk '{print length}'
4
$ echo asdf | busybox awk '{print length}'
awk: cmd. line:1: Unexpected token

it does support the implicit $0, but only when () are used:
$ echo asdf | busybox awk '{print length()}'
4