No, in 2006 it was still considered poor form to reply with a low-effort meme phrase instead of meaningfully criticizing his position with your own.

Depends entirely on the forum.

I remember it being somewhat common for people to make forum posts consisting entirely of a joke image. However, they weren’t called memes at the time as the word had yet to be popularized.

It was not correct to interrupt a talk with a meme and it never has been.

But it's always been correct to interrupt a discussion on a PHP forum about PHP security by breaking into Rasmus's account and posting an ironic meme under his name.