Then I discoverd ,what was news to me, that his wife was Tom's niece.

赵俊英显然是按照现代语法分析这个句子的。因为传统语法并没有评注性状语（disjunct)这个术语. 该术语是夸克提出的。传统语法则简单地称这个what从句为插入语。

根据夸克《大全》，adjunct，subjunct, disjunct 和 conjunct是从句法上对状语进行的分类。而从语义上，状语则可分为时间状语、原因状语.....、除外状语、比例状语、评述状语等。

其中，评述分句 comment clause 是对状语分句的语义分类。在语法结构上，评述分句属于插入性disjunct (Comment clauses are parenthetical disjuncts.）。

需要注意的是，现代语法认为，主谓宾状等成分的划分是在分句clause的层次上进行的，而不是在句子sentence的层次上进行的。所以评述分句作为状语从句其本身是个分句成分，而不是句子成分。评述分句是说话人对自己说的话的评论，即作为分句，它是对它所附着的分句的内容进行评论。我们先看下面句子：

What was news to me, his wife was Tom's niece.

(= What was news to me was that his wife was Tom's niece. what从句作主语。)

(= His wife was Tom's niece, which was news to me. which从句为句子关系分句。)

很显然，作为评述分句，what从句是说话人对主句his wife was Tom's wife的评论。（注意不要用“修饰”这个术语。评注性状语并没有修饰功能。）

现在我们让 What was news to me, his wife was Tom's niece.这个句子作discovered 的宾语从句。陈述分句作用作名词性分句时需要由that引导。如果名词性that从句本身被状语从句修饰，that放在什么地方？这就要看该状语从句在句法结构上属于哪种状语。修饰性状语（adjunct)在结构上与其主句是一个整体，即结构上修饰性状语从句是主句的一部分。所以，that必须置于状语之前。例如：

I know that if we heat water to 100 ℃, it will boil.

评注性状语则不然。评注性状语在结构上不属于主句，是外围性结构（peripheral）。what从句作为评述分句，必须置于它所附着的分句之前。结构上它又不属于该分句，即that不能置于what分句之前，只能置于what分句所附着分句之前。于是就得到了下面的句子：

Then I discoverd ,what was news to me, that his wife was Tom's niece.

语法结构上，that从句为discovered的宾语从句，what从句为that分句的评注性状语。语义上what从句是that从句的评述分句。

所以，what从句作为评注性状语，是说话人对that从句内容的评论，与主句 I discovered无关。赵俊英说评注性状语“修饰”整个“句子”，从现代语法的角度来看，这种说法有二个不严谨之处：1 评注性状语并不修饰主句。修饰主句是adjunct的作用。2 评注性状语修饰的是主句 main clause, 而不是句子sentence。

本句中，what从句评论的是它所附着的that从句（that 从句是what从句的主句），而不是评论整个句子sentence。

题外话，我觉得这个句子稍作修改，读起来更为通顺。

Then I discoverd what was news to me — that his wife was Tom's niece.

用what从句作宾语，that从句作what的同位语。

Then I discoverd that his wife was Tom's niece, which was news to me.

what从句改为which引导的句子关系分句。