idbeholdME: It's not about content, but about differences. Different single player campaign, weapon differences, balancing, movement AI behavior, missing bot name voice overs in 2004, menu, UI and more like the fixed "boost jump" bug from UT 2003 which got fixed in UT 2004. All of which absolutely warrants UT 2003 as a standalone entry. I play both occasionally, so speaking from experience.
I have no personal experience to compare to, so must take your word for it.
It does seem rather strange, that they state 'containing most 2003 content.', if the single player campaign is totally different. Because as you say, that alone warrants a separate release, and then add to that the other differences you mention, it is truly weird. But in truth, Epic have behaved weird all round with Unreal in recent times. Maybe they are working up to something big, that creating rarity will enhance.
EDIT I've just realized I have something at Epic, obtained for free back in August 2019, called
Unreal Tournament - A NEW BEGINNING. I presume I can still download and install that.
While I have discs for
Unreal and
Unreal Tournament, I also have
PC GAMES__Quake3 vs Unreal Tournament …Issue 21, which features a lot of extra Unreal stuff. I actually have heaps of Unreal stuff spread over many PC magazine discs.
I of course have all the Unreal games that GOG had.