It is dialectically true that "social construct" and "figment of the imagination" are two different things. It is rhetorically false; in rhetoric calling things "social constructs" is clearly an attempt to simply label them as figments of the imagination.
Huh. You really believe that people who say that race is a social construct are trying to label it as a figment of the imagination? That's not my intent, at least, and I doubt very much that it is the intent of anyone who makes that claim in good faith. I'm not trying to argue using rhetoric; I'm saying that if one tries to segment human variation into races ignoring societal attitudes, one will either fail entirely or will arrive at "races" that do not correspond neatly if at all to society's conception of race. That is the sense in which race is a social construct.