You don't know Glenn. He's a natural story teller and while proud of what he's done, he recognizes that he's just one of many people trying to create something that hasn't been created before. In reading into his comments of his history with Steve, you're viewing him as intentionally pumping up his own story. The truth is he's trying very hard to be honest with the listener as to his comings and goings with Apple and with Steve.
He worked with him closely, then stepped away and came back. Multiple times. By telling the story of this, he lets readers know that he wasn't always by his side and that he left to try other things but circled back at different junctures in his life and in Steve's.
Knowing Glenn and his successes and failures and talking with him over many a margarita at Compadres (after long bouts of writing NeXTSTEP code with him), the story brought strong memories and a very clear voice of different periods and circumstances across several years. He shows an arc of ups and downs in both careers and a lot of hard work and sweat that often didn't get notice until many years later if at all.
Rereading the story, I don't get boastful at all. I get much appreciation for being able to be on a pretty cool ride and being able to create some great products. There's also disappointment in some hard work that didn't pay off. But above all appreciation for someone who continually rolled up his sleeves to get in the ring, take chances, and build something... or in this case tell a damn good story.