The research you've found is fascinating to me. I didn't realize 4 years was the ideal age gap. Yet I also think there are likely plenty of exceptions to your theory that eldest children with siblings close in age may be more likely to be emotionally detached, frustrated, socially awkward, insecure, and angry.
My own experience, my husband's experience, and my kids' experience are definitely exceptions to your theory.
As the eldest child who is 14 months older than my younger brother, I am similar to your husband when it comes to birth order and age gap (although it sounds like he had 18 months between him and his sibling).
Yet I'm VERY different so I do think inherent personal traits can overcome birth order/birth spacing. Despite early memories of being jealous of my brother (I literally remember crying because my mom was holding him and she couldn't hold an infant and a toddler at the same time), we have a very close relationship. I am also an extrovert who is very comfortable sharing emotions and regularly counsel people in crisis.
Yes, I've always been the responsible one (e.g., I never got detention, was always a teacher's pet) and the risk-averse one (e.g., my brother quit his corporate job to go to a start-up!). Yet I have never been emotionally detached, frustrated, socially awkward, insecure, or angry.
My husband is more introverted than me (he's also the eldest, with a sibling 2 years and 8 years younger than him). Yet he is very good at chatting up strangers (he just needs time to recharge after a long day of talking), talking about feelings (he's more empathetic than me!), and confident than me.
My eldest daughter is more extroverted than my younger daughter, but they're SUPER close and talk about having "sister-time" all the time (and they're teens).
As I wrote in my story here on Medium, my brother and I were so close that he told the store employee that his name was "Anna-David" when he got lost in a store as a preschooler.
Have you floated your theory with your husband? Does he resent his younger brother?