It's actually a rising pH that favors tannin extraction, most often quoted as >6. There's no way you extracted tannins if you did a thick decoction as your pH would have been a good deal lower. Now, if you oversparged (unrelated to the decoction) with too hot sparge water, that could lead to tannin extraction.
Tangy and/or sour wouldn't be an indication of tannin extraction, anyways, which imparts more of an astringency than sourness. I think your beer will be fine.