Aside from that picture there's a couple of other things:I think they meant a different theory: That Calise may be another of Kas' disguises which is based on data-mining for some stuff that's not currently visible in-game. I don't buy it myself because it's mainly based around one piece of art showing her with demonic eyes, which we know you can get in other ways (and there's art of Futa!Calise with normal eyes as well so I think it's just meant to be a sign that she can become corrupted) but I can see where it's coming from.
The Blood Knight deals Holy Damage, which is weird for a demon.
Calise's backstory fits with Kas'
Kas refuses to show her original human form, "for now".
Calise's lines are "warmer" when on Kas' romance path.
Admittedly, it's all circumstantial.