Full Idea
Verificationism about concepts implies that thinkers will not share concepts with adherents of theories they reject. Those who reject the phlogiston theory will not possess the same concept as adherents, so cannot say 'there is no phlogiston'.
Gist of Idea
Verificationism about concepts means you can't deny a theory, because you can't have the concept
Source
David Papineau (Philosophical Insignificance of A Priori Knowledge [2010], §6)
A Reaction
The point seems to be more general - that it is hard to see how you can have a concept of anything which doesn't actually exist, if the concept is meant to rest on some sort of empirical verification.