Exactly. If you can only see this person as romantic interest even if your relationship is just friends, then it is difficult to maintain a relationship as friends because if feelings grow, then friendship is not going to be enough. And people who say they will always be just friends sometimes underestimate time and the random nature of relationships. You can be completely disinterested in someone initially but as you get to know them, you may develop feelings. People are so conditioned today to think attraction must always be immediate or instant or strongly felt initially to be real that they underestimate how relationships "develop". You can develop feelings for someone just as someone who you were once passionately in love with may later be seen as a former lover or a friend. And people change. People can develop qualities or traits that make them more attractive to someone or desirable. The assumption that the feelings were always there, latent, until they come to the surface denies the reality that feelings develop without any intention on a partner's part. Just because it's not love at first sight, doesn't mean they can't be that great love or THE love of your life. You can't always tell how you're going to feel unless you're in it and things unfold. If couples always predetermined that someone should be nothing more than friends without knowing how the relationship would progress, there's the chance that many of us wouldn't even exist.
People grow and change their minds. That's the cool thing about real life and free will, it's always changing. And you just never know.
So, the question, in my mind, isn't whether they can be because of course they can be just friends. Not everyone man and woman has romantic attractions toward each other. The question is, whether they will be.
Two people can be brought together by unusual circumstances and develop a love and trust for each other that creates more personal connection, bond, and committment.
How many people have said, " I would never date A or B because I don't see them that way" only to marry them or have children with them later on.
Not every relationship will or has to work out. Who knows, maybe one or both people know that going further may not be the best option. Hat's off to them for figuring this out early. Avoid predetermining. You never know.