We hold these truths to be self-evident, that all men are created equal
我的理解是,这个equal是主语补足语,不是状语。就是说强调 all men are equal这一层含义。而且这个equal是与生俱来的,这个句子应该相当于all men are equal when they are created. 类似的例子,可以参考
Her brother was born blind. (牛津高阶第九版)
He died happy. (韦氏高阶双解2017版)