とはいえ＝といっても (approximately), meaning "although."
It's similar to 〜くあれ＝〜くても or ＡあれＢ＝ＡあってもＢ, in that it's a written-style usage of the imperative that functions as a concessive. That is, it's conceding something: although, though, even though, yet, however, etc.
春【はる】とはいえまだ寒【さむ】い。 = Although it's spring, it's still cold.
Keep in mind that this isn't 言【い】う in the sense of "he said ___," but more in the broader sense of defining and describing a concept. As a general point, while you're correct that いう is 言【い】う, the two spellings imply different usage. Although it's not a rule, generally 言【い】う will refer to an action, like something directly spoken:
何度【なんど】も言【い】った。 = I've said it several times.
いう, on the other hand, generally used in phrases like という or こういう, has the broader senses of defining, describing, quoting, emphasizing, etc. It implies a grammatical function (like とはいえ):
日本【にほん】という国【くに】 = The country (called) Japan