linux command whereis returns the path of a non-executable file

Help me understand the logic, please – from whereis man:

OPTIONS
       -b
           Search for binaries.

But when i use this it returns path to dir:

kl@debian:~$ whereis -b postfix
postfix: /usr/sbin/postfix /usr/lib/postfix /etc/postfix
kl@debian:~$ whereis --version
whereis from util-linux 2.38.1

kl@debian:~$ file /usr/lib/postfix
/usr/lib/postfix: directory
kl@debian:~$ file /etc/postfix
/etc/postfix: directory
kl@debian:~$