Well, we don’t know she’s a lesbian! She could very well be bisexual. (Ymir, on the other hand, bluntly agreed she had no interest in men, so I think it’s safe to say she’s a lesbian.) That being said, Historia also has not explicitly canonically shown interest in a man, so she might well be a lesbian. 

But yeah, Yumihisu is canon. It’s like denying that Ash and Eiji in Banana Fish have romantic feelings for each other–they certainly love each other in a way that is almost spiritual, in a sense, and their love has many facets, but Historia and Ymir, like Ash and Eiji, absolutely have a romantic component to their relationship. To say otherwise is to misread the text. Historia and Ymir never kissed that we saw, but she thinks of Ymir whenever she needs inspiration to keep living (like in the cave). In contrast her current relationship with Farmer-kun is like… miserable. She’s miserable. Historia can of course move on and find someone and be happy, that’d be beautiful too and I don’t fault people who want this, but Yumihisu was definitely a thing. 

But Ymir has been out of the plot so long, it’s understandable there isn’t much content right now. I really hope Ymir’s memories via Porco play a role in jump-starting Historia’s positive development again.