I keep saying the same thing over and over, but I think my favorite part is that Eri didn't just go "oh I do actually love you, after all! It's totally the same kind, and I realize that now after all that hard thinking I done did."
Nope - she's fully aware that she doesn't understand the exact type of love that Fuyuki has for her. But, she understands that it's there and that it's real, and that she and Fuyuki care about each other just as much, if not in exactly the same manner.
And to reiterate on the same point I've made elsewhere - I think that might be good enough, in their singular case.
Fuyuki makes Eri's world brighter. Even if Eri never actually initiates a kiss or an "I love you", she'll respond completely to Fuyuki whenever the latter does or says the same. It might be less important to "label or define" what Eri feels for Fuyuki, than it is for them both to understand and internalize that the label or definition is secondary to the fact that the intensity and depth of her feelings for Fuyuki are undeniably real.
And I think that their decision to pursue something together, even if it might hurt them both at some point in the future, is beautiful, and authentic to their characters this whole way through.