If you really try you can twist any romantic action into a platonic one, by virtue that both are very similar with the only true difference being the emotion behind it. Which you can usually not tell unless the character states it externally or internally. You can in fact go completely off the rails and claim a kiss on the mouth or any other sign of physical affection are platonic. Just an expression of appreciaton. Saying "I love you" doesn't mean romantically. Aromantic people may do these things sometimes. Therefore we can never be sure, right?
If someone is stubborn and ignorant enough not to read between the lines, not to understand the mood or narrative theme of what Kasumi's feelings are about... then they truly can only be satisfied with sexual themes being introduced. That is the only conclusion one can take. Sexual interest is not platonic, so it would be the unavoidable confirmation of romantic interest too, right?
Except it isn't! Might as well go fully mentally challenged! Because people can be aromantic, but still have sexual desire! Then everything is quite clear, isn't it? If Kasumi ever shows a hint of sexual attraction to Sakurako, she is still aro and has only platonic feelings for her. If Kasumi shows romantic affection, it is simply pretending or the flipped version of a platonic/familial love. If Kasumi shows platonic love that just confirms what is inevitable already.
Completely logical. Superbly reasonable. Kasumi MUST BE aromantic. It is the only way. Because no matter what evidence is given, unless she says with her own mouth and the straightest of faces "I , Kasumi, have romantic feelings for Sakurako Kawawa of which I am absolutely sure and which I will express to her firmly without a hint of dishonesty or humor." there can never be certainty.
goes to drink self into a coma