Little did he know,  Apple would eventually balloon to become the world’s most valuable company, the first to reach a covet.

网友的理解是正确的，Little did he know是插入语分句，其后为主句。

如果改为Little did he know that...则little did he know为主句，that引导宾语从句。二个句子的区别是，主句表示主要信心，从句表示次要信息。

如果说话人想突出Apple would...分句的内容，就将Little did he know用作插入语，Apple would分句为主句。

如果说话人想突出He knew little，则用其做主句，Apple分句作宾语从句。

二个句子的意思相同，只是信息的侧重点不同。