ADHD here, which is tangential to the spectrum, but I've also been said to have high-functioning autism at one point in my life by a professional. Can confirm, his behavior feels familiar. I was also extremely slow and dense emotionally during my schooling. I never developed an overt crush at the time (my first crush was in my mid-20s, eventually panned out into a legit relationship for a few years in my late-20s), and my high school years were spent surrounded by dudes in an all-boys school so never socialized with girls regularly around that time, outside of my sisters (I have three older ones) and their friends.
There was one time when a girl classmate from the combination elementary-middle school I went to invited me to a 'mixer' dance at her high school, and spent the evening with her and a few other girl classmates I remembered from the same school, but that was about it. Looking back, I kinda wished I had been more accepting of her boldness to ask me to that dance, and maybe tried to reciprocate the apparent feelings with some dates afterwards. I only agreed to it under the idea we were going as friends, but I feel like I could have at least tried to see if I could feel the same as she likely did. Missed opportunities.
To be fair, there's more factors than just autism and ADHD to my lack of emotional development- there was a failure in paternal rearing, for sure. My sisters and I suffered a lot. But I've already rambled enough about things in a random comment thread. No need to go on about family drama.
Anyway... is she sure she didn't jump on him on purpose? I kinda feel like she doth protest too much. I mean, sure, it was a reactionary thing, but I think her subconscious clung to him instead of the friend on the other side for a reason.
Also, I think he's overestimating how much Shia actually sees. The narrator has been proven unreliable. Just because he thinks she's very observant doesn't mean she actually is- she might have been paying special attention to him and what he's been wearing because it's him. But It's not like a character to jump to that kind of conclusion about their own chances- that'd be too convenient. Though it's not like he needs that to shore up courage- he's been confessing that he likes her without much issue, it's just she still doubts he really likes the real her, or all of her.