You ought to have given them some advice. 你本该给他们一些建议的。

— _________, but who cared what I said?  —— 我给了呀。可谁在乎我说的话呢？

别人以为你没有给出建议。

你的回应是我给了呀，不是没有给。并不是我确实应该给。

所以So I did. 意为 I did so. = I did give them some advice, but...

你误解了句义，所以才有了疑惑。