I think yes it is generally considered stalking. But he didn't do it in a creepy way the other guy did. He didn't spread rumors Abt yidam that would ruin his social life. He isn't doing other creepier stalker behavior like in 'romance but not romantic' and some other creepy ASF stalker stories
He just gives off the vibe of a kid who has no idea what to do Abt his first love. Esp bc his first love had no idea who he was. He didn't know the first step. Like yidam said. Sunwoo never made him feel uncomfortable. Sunwoo only ever prioritized yidam. And I really do think the incident in the early chapters(ch 3 when sunwoo gets his heat) was an accident somewhat caused by yidams pheromones. So while ye he's a stalker. He's the best kind of stalker lmaooooo

Bro is so me except the stalking part later on yk