Yep, basically “I agree with this statement a lot.” It’s very much an “online Americanism.”

In the superlative, yes. It's a fairly new phrase, and hardly in my parlance, but it's growing on me when I'm in informal typed chat contexts.

It's a call for others to take note of the important or profound message being highlighted. So more than just "I agree".

When someone says "this" they are basically pointing at a comment and saying "this is what I think too".

"So much" is applied to intensify that.

So, yes, it's a strong assertion of agreement with the comment they're replying to.