Yeah I get what your saying haha. I just take the word modern at face value and use it to encompass all metal that comes from the current age we live in. I can see how certain 'modern' bands though would still be called classic metal. Especially the band Skull Fist. When I first heard them I thought I was listening to something from 1981, not 2011 lol.
This a relatively recent band that play "oldschool" metal(like having a classic 70's /80's metal influence and sound), I wouldn't call this "modern" metal, modern metal is more like metalcore, deathcore, and that stuff, but that's just me...
In my definition modern metal is just modern heavy metal, the NWOTHM in particular. Seeing as that whole movement in and of itself is proof that heavy metal is immortal.
It's just that most people who(not all of them tho) listen to this type of metal call modern metal not to new bands that plays metal(different to "nu" metal...see what I mean?), but specifically to bands that play metal but in more "modern" approach instead of the more traditional or classicalheavy metal. In the end it's just a matter of perspective I guess, for I, I just used to differentiate styles of music rather than the antiquity of the bands that play it.