OK this is of course not about you but I am starting to get somewhat annoyed by the subject, you are correct it would be sarcastic, with the sarcasm I have demonstrated two ways in which "how much" can be used to mean the opposite.

in the first one would say: I am surprised by how much you know, which is very little indeed.

the second is the sarcastic one I made that you noted above.

Of course sarcasm is generally conveyed by tone. One can make the sarcasm completely deadpan. Now someone has to evaluate the text to determine if "How much you know" actually means "you know very little", my friends would know I meant that the person did not know much when I said how much, someone else might mistake my meaning, but really, probably not. Because in the context who actually would respond in that way?

But let's go back to the original statement I made that how much and how little are technically equivalent and everybody being so peeved at it. Because there are some other ways that we can use the phrases that show their equivalence.

"hey you have to pay for that"

"how much or rather how little?" there is of course a nuance, by saying how much I am asking what amount I have to pay, and when I follow with how little I am asking the amount I have to pay but asserting I expect that it should be a minimal amount.

"hey you have to pay for that"

"how little?"

I am asking the amount but communicating I consider whatever amount it could possibly be to be so minimal it's almost not worth discussing. Obviously here we have another place where the common "how much" would almost always be used, as "how little" would be somewhat annoying and maybe announce that one is a bit of a show-off.

Someone earlier made the comment about that English is very context-dependent, indeed it is, the original "how much" usage was so obviously being used in a way that it meant what would commonly be said "how little" that someone actually asked "do you mean 'how little'". Because of the various ways that much can refer to a small or indeterminate amount - for example

"you have to pay for that"

"how much"

"one penny"

"how much is one penny"

"one penny is very little"

"how little"

"it is the smallest coin in our currency"

it should be clear that the phrases "how much" and "how little" while having common usages and meanings these are still very much determined by their context, and in the response to the original phrasing I think, given all the ways they can be used, that it would have been more reasonable to ask not if he meant "how little" but assume that "how much" was being used to describe something small, which is one of the ways it can be used, as demonstrated (albeit not exhaustively) in various scenarios here.

All of which comes back to the example I also made earlier that as comparative statements how much and how little are only really tightly locked when used together, as in "How little do I get for how much!?"

on edit: obviously smallest coin in this context means coin holding the smallest monetary value.