Yes, but Kasumi said she just prefers to be Sakurako's pet cat. In the end it's up to the people participating in a relationship to define it. In many cases even each partner will view their relationship differently let alone what strangers and various other onlookers think. Still, final word goes to the people in said relationship.
If I see a girl and a guy eating at a fashionable cafe together and acting extremely close to one another I might interpret it as a couple who are dating, but if they tell me they are just siblings hanging out than that's what they are.
Anyway, the entire point of this chapter was that not all relationships fit into silly pre-cut categories or labels nor should they.
This would only apply if we had no insight on their general life though...
I want you to repeat this sentence out loud and then smack yourself against the head:
"We cuddle and sleep together, romantically love each other, kiss each other, go on dates together, live together, want to get married, are already integrated as members in the respective other's family and base every decision of our life around each other. OH but we are totally not dating though."
Heck I smacked myself for saying that out loud. As explained before, people can make up or ignore definitions and naming coventions if they want, but on an empirical level, its just undeniable.
Seriously most people would stop being in denial at the "love each other romantically and go on dates" stage.... How do you do it Yukiko-sensei?