That word isn't always used as a negative, but rather to emphasise. We English are terribly strange. While it does actually mean how it sounds, he/she may not have meant it quite in the way in which you took it. I think we tend to forget, sometimes, which adjective we're using, and why.
One liter = 1 liter = 1 L
All of these should be accepted as correct, 1 L is currently marked as wrong.
PS. With the exception of degree, (°), minutes (´) and seconds ('') there should always be a space between the number and the unit.
Source: International Bureau of Weights and Measures, SI Brochure, §5.3.3