I was just about to say that too. Zdoom is more of an open source engine, while skulltag is.... well, not that much open source.
Personally, i don't get why it has to be named after doom. Why not strife,hexen and let's not forget the best one Heretic! But i guess if it would be called zheretic i would ask why it isn't called zdoom.
Because Doom is Doom.
No Doom = No Strife, No Heretic, No Hexen.
Doom rocks, because it's just it Doom!
This doesn't make any sense.
Anyway, ZDoom is called ZDoom because it started as a Doom port. We're talking about April 7, 1998 here. Heretic and Hexen support was added progressively afterwards, first from reverse engineering and then, once the code was released, by importing it. For example, the actors and their states were added in March 1999 (and even then, not all of them). Loading the Heretic and Hexen IWADs was only added in February 2000. Support was mostly complete around 2003, though there were still many bugs and some features (such as the loading screens) are a very recent addition. Minor actors were still not all implemented by this time, for example it's in 2004 that the Hexen dripping water actor was added.
It's in 2003 that Strife support started being added. The Strife teaser was fully playable in July 2004, with the full version following soon after.
Chex Quest support was added in 2008.