Lime production is carbon intensive (heat + chemical C02 release from the limestone) and the hempcrete curing will take 50 years to fix back the second part. So it can be carbon negative (by fixing the carbon of the hemp itself) but only on the long term (50 years after production) and assuming that the heat used for lime production was from non-fossil source in the first place (which is not the case today: the cement industry typically produces heat by burning natural gas).

The heat could be produced from burning green hydrogen (hydrogen produced by water electrolysis using renewable power) but as of today is still too expensive compared to using fossil fuels.