You are completely correct, but Kamoi didn't really want to date Rion, she just wanted to maintain their close friendship and was willing to play girlfriend/boyfriend to make that happen. As she said, it would give them an excuse to call/message everyday, to visit each other on holidays, etc.
(and being able to run from dealing with her feelings for Stowie would have just been a nice bonus)
And again, you'll be completely correct asserting it is a very bad decision that feels good in the moment but would lead to long term issues. But these are highschoolers and that pretty much describes nearly every decision you make in highschool.
Rion also turned her down because he loves Kamoi but knew it would never work out between them, they were simply too close to be able to date each other, and after having some cope-boning with his cram school tutor got the perspective to know that what she was suggesting would absolutely kill him.