conjunction
even so; even that
Concessive conjunction used to say that even the thing just mentioned does not change the situation. Often followed by a negative or limiting statement. Somewhat literary or formal.
彼は努力した。それとて結果は変わらなかった。
He made an effort. Even so, the result did not change.
金があれば幸せになれるとは限らない。それとて同じことだ。
Having money doesn't necessarily make you happy. The same goes for that.