He didn't know what it meant until it was explained
I think the significance is in which finger it's tied to and what it means? So he saw the string in the 2nd chapter but he still doubted Aoba's love and how serious it was. The relationship god explained that if they're both tied by the pinky it means they truly love each other--and since Haruma *did* see it that way from the second chapter, he realised that it meant that a) Aoba loved him from the start and b) his feelings for Aoba are also real. (The dream in the mirror also helped to prove that Aoba cared for him for years.)
That's how I read it anyway :).
The string was always tied (tied, not wrapped around) to Haruma's finger (proving that Aoba loved Haruma from the beginning) but it was only wrapped around Aoba's pinky because Haruma didn't love Aoba in return. It wasn't until chapter 3, page 15 that the string was tied around Aoba's fingers. That's when Aoba noticed the string being tied around his finger (proving that Haruma has returned his love), hence why Aoba was so happy and kissed him.
Haruma didn't know the significance of the string being tied or that there's a difference between the string being 'tied to the finger' instead of just 'wrapping around the finger' until Shiro pointed it out.
as a short version, he actually had a wrong idea about it. he thought it was the god's doing but yeah, as everyone says, he found out that it was related his feelings all along.
Oh I get it nowww! Thank you so much for providing time to answer my question! [tight hug] ( ´ ▽ ` )
Why was Haruma so shocked at the string that was tied at the end of Aoba’s pinky in the 3rd chapter when Shiro pointed it out? He already saw it in the 2nd chapter right?