I think I found the line you are referring to, on line [958 here].
I guess I can try patching it so that line is removed just for testing.
(https://git.busybox.net/busybox/tree/miscutils/crond.c)
log8("USER %s pid %3d cmd %s",
Looking into it further, setting log level of 9 probably did not make any difference, as it only has support for 5, 8 and 9 (possible bug on level 5, as it is actually set to 4). This is on lines 188 to 210.
I might be able to make some improvements here in the form of a patch by implementing support for a higher log level.