Like, definitely fake, but I wanna talk about Artorias for a moment.
Thing about Dark Souls is that the mc is a real non-character. It's not just that they're a blank slate, it's that their not remotely the memorable part of the series. The memorable thing is everybody else you meet, like npcs such as Solaire or bosses like Artorias. And I feel like this isn't unprecedented for Smash. Look at Monster Hunter. Everybody thought they were pretty likely for base game, but instead we got Rathalos as a boss. Part of me think that's because the dev team realises that what's memorable about MH isn't the generic hunter, the real icon is the monsters themselves. If that is the logic used, then Artorias could fall under something similar. He's easily at least top 3 most popular character in the series, with his main competition being Solaire. Thing is, Artorias is more important to the lore, is arguably more emblematic of DS as a franchise, and is more 'recurring' throughout the series, albeit in name only.
On Dark Souls itself, I've always thought it was a dark horse pick for dlc. Honestly, at least from my perspective, Dark Souls is probably the single most influential japanese game ip created in the last decade. It's had a very noticeable impact on games, has become super well known as a series (at least relatively) and the IP holder directly works on Smash. You wanna talk about 'iconic new blood' for dlc, look no further. I wouldn't argue it's very likely by any means, but it makes a lot more sense then most people realise. Could happen.
Oh an yeah, again, this leak is definitely fake, but a DS character could get in on its own.