could be said to 不是插入语，而是一种提升主语结构。所谓提升主语结构，是指从句的主语提升到上一级结构中（即主句）作主句，从句的谓语动词改为不定式作主语补足语。例如下面句子的变化：

They say that Tom has left. 这个句子中that从句为宾语从句。这个句子改为被动语态则需要形式主语it：

It is said that Tom has left. It为形式主语，Tom是that从句的主语。现在把that从句的主语Tom提升到主句作主语，则为Tom is said 此时is said的语义是不完整的，必须补足is said的语义，对主语作进一步说明。于是that从句的谓语动词has left 改为不定式 to have left, 得到下面的句子：

Tom is said to have left. Tom为提升主语，to have left为主语补足语，补足is said的语义。

网友的这个句子中就是提升主语结构，只不过主句谓语带有情态动词could而已。我转换如下：

They could say that I wanted to write books at that time. 他们/人们可以说我当时想写书。

It could be said that I wanted to write books at that time. 可以说我当时想写书。

I could be said to want to write books at that time. 我可以说当时想写书。