Porn explicitly depicts genitalia or sexual acts showing genitals. Nipples don't count. We'd see the expected censorship bars or mosaics due to Japan's obscenity law.
Since the art doesn't show that, it doesn't count as porn. Instead it's smut or erotica; still not okay for minors but able to get published or distributed in more places.
The next step down has a wide gray area. Some publishers don't allow for exposed cleavage, others allow a single flower petal covering the naughty bits.
The definition of what is Pornography has been changing in the US btw, some states generalize it as anything that can cause arousal.
