I think it does mean per square metre, notwithstanding that I'm somewhat mathematically challenged. That's what it looks like from some symbol references I found anyway.
Sounds like a lot but according to the full text of the research the grounds used had 65% water content, so that would have constituted a significant portion of the weight. They used a lot less than that in some of the tests. FWIW I just weighed the spent grounds from our coffee machine and just that small amount in the filter came to 110 grams wet weight.
