That is not the point really. It is more so about friendship being seen as lesser or insignificant next to romance. At it's core, he is most afraid of not being "important" to her. And probably due to biases about relationships, especially those between men and women, he thinks that the only way to stay important to each other is if they're romantically involved. Which is a lot of internalized misogyny and homophobia, as he was literally comforted by the fact that his friend is seemingly only interested in women, which "dooms" her to never having her feelings reciprocated, leaving him labouring under the delusion that if he's there for her enough, he'll eventually get to win her as a romantic partner.
Her talking about how he has a place by her side is breaking that preconception in his brain, ensuring that his friendship is valuable even when she's dating someone. Only when he gives up viewing her as only a romantic prospect and instead decides to view her as a person/friend is he finally able to genuinely be happy for his friend when seeing her happy. The oneshot is ultimately about this boy being pulled out of his mindset into one that allows him to actually care for and be happy with the people who are important to him.
