relate：(formal) to give a spoken or written report of something; to tell a story 《牛津》

例句： 

She related how he had run away from home as a boy.

The story relates that an angel appeared and told him to sing. 这个句子改为被动语态则为：

It is related that an angel appeared and told him to sing.

改为自由间接引语结构则为：

An angel appeared and told him to sing, it is related.

所以，网友的句子就是一个自由间接引语结构，引述分句it is related以插入语的形式出现。

Only a few years ago, it is related, as soon as envoys sat to dine, “stealth boasting” would start. 改为间接引语则为：

It is related that only a few years ago, as soon as envoys sat to dine, “stealth boasting” would start.

The story relates that only a few years ago, as soon as envoys sat to dine, “stealth boasting” would start.