Yea I kinda noticed this myself. Theres also a few people saying how it wouldn't be Frostmourne if reforged ignoring the fact that Ner'zhul and Arthas are both gone (Metzen himself specifically said Ner'zhul's story as the Lich King is done) and Frostmourne was plenty powerful before Arthas starting soul stealing with it.
And then there was Shadowmourne, which is explicitly stated to be a sister blade to Frostmourne. The only difference between the two was that Frostmourne had a LOT more souls empowering it at the time, and Shadowmourne didn't have a pissed off Orc spirit linked to it to corrupt whoever wielded it.
Shadowmourne would be cool, but Blizzard has said that they aren't re-using old weapons for artifacts so its already been ruled out.