That's true and a good point, but not (it seems) applicable to this particular function, which validates whether or not the name is one of two fixed-sized formats, right?
It looks like what's going on is that gethostbyname() calls __nss_hostname_digits_dots() which checks to see if the string you passed it was an IPv4 or IPv6 address rather than a name, and in that case it functions like inet_aton/inet_pton and converts the IP address string to a binary IP address as though the "name" 1.2.3.4 resolved to IP address 1.2.3.4.
In that specific case there are no aliases and exactly one IP address, but the buffer could still be too small (e.g. if caller-supplied with gethostbyname_r()).
(edit) You may be totally right here, by the way.