After the confusion once episode 10 aired, the original author of the manhwa posted to Twitter/X and explain the intent.

Author Iiromill (이로밀) clarified that the series' events are a true reality rather than a dream or illusion. The final classroom sequence represents Eun-ho's internal reverie and subconscious processing of his past regrets, letting him envision a scenario where he confessed to Do-hyeon much sooner. This imaginative transition highlights Eun-ho's emotional growth and breakthrough, preparing him to face Do-hyeon's real feelings openly without erasing their prior relationship.

Essentially, Eunho falls asleep and dreams of those early years when he was hesitant and insecure in his feelings for Do-hyeon, and this dream allows him to take a different path and choose what he wishes he had done now that he's embraced his feelings and is secure in their relationship

This was made a little confusing because of how the dream was presented and lines like "So it was a dream", but ultimately the sequence was merely meant to highlight his internal growth and emotional arc as a character.

The ending with them on the picnic blanket is where we leave them in real life, happy and together and fully embracing their new relationship without insecurities.