You say no one write that unless they mean it. Yes, they do.
Here's a story I do not often share: before we got engaged and married my husband left me. One day he just messaged me (yes, messaged me) and said "i dont think I love you"
My entire life and world fell apart. We lived together and he was at work. I had to pack up all of his things and take them to him and he dropped me home (I had his car, mine was broken at the time). When I asked why, all he could muster was that he didn't think he loved me. He couldn't say if he ever had. His mother called me, in tears saying that she knows he loves me and she will make it right and that regardless she loved me. His father could be heard in the background shouting "imma kick his head in when he gets here for this, he's a bloody idiot"
He didn't make it home. He came back to me and said, "i don't know what I want but I know I don't want to be without you." we stayed in this weird relationship limbo for a couple of weeks, until I cracked it and said "choose! You are with me or you're not. Either you are all in or I'm out." he had 1 day. He actually sat down and thought about everything. He realised it wasn't that he didn't love me, it was that we had gotten comfortable, we weren't exciting. I had swapped degrees at uni and had been through upheaval that included a threatened law suit with the uni. He thought I had changed, but he realised I hadn't. He came back to me trying to propose. We took another 6 months before we did get engaged.
It damaged our relationship for a while but we worked through it. We are strong now but that part of history will always exist. It sounds to me like your SO went through the same cold feet, but rightly, didn't execute the manoeuvre until he was sure and he determined he didn't want to send it.
Tell him you found it and talk about it, otherwise he won't get to explain it will eat you alive.