Yikes...

Yeah... I was like 'oooh, boi... You sure you wanna make that promise that easily?'

I think what this chapter made pretty clear is that they are not together for convenience. Their confession included a whole dialogue about how much they thought about if the feelings were real ... let's give the author credit for making their feelings clear.
Plus, these two characters are the kindest in the story and the ones to be shown to be less selfish and concerned for others so leading on someone into a relationship for convenience saying they feel more is just out of character for them.

I don't find hard to believe these two fell in love for each other ... since they met, it seems they had a natural bonding that allowed them to be very close. They were both in love with people who were not their match and in every scene Junwoo x Sarang are together the first thing you think is how much they fit in personalities. Plus, the fact that then shared each other's secrets and problems ( Junwoo's unrequited love, his family, Sarang's problems with Eungi, his family) made for a connection that they didn't had with others. And they are both sweethearts, attractive, kind ... so how could they not fall in love with each other?

I say convenience because if you go through the same abuse Sarang went through then have a friend who is a okay person.
Jinwoo who been emotionally stomped on repeatedly then see's someone who is his type and an accepting, sweet person

Naturally you'll push the affection you've been yearning for onto someone else who doesn't seem like they'd hurt you. And that's what it looks like the two are doing subconsciously, which is why I say convenience.

I will give the author credit for clearing up the Jinwoo isn't using him as a dupe for Woojun because that's an obvious concern.