That the poem is one work of romanticism shown not only in exaggerating symbols but also in its flowery words.
请问句子一开头的 that 是不是多余的?
同意你的看法。The poem 是主语,is 是系动词, one work of romanticism 是表语,shown not only in exaggerating symbols but also in its flowery words 是过去分词短语作 romanticism 的后置定语。