(1) They were definite alibi witnesses who could place DW elsewhere when crime committed; or
(2) some other category such as persons who would flatly contradict, falsify, and impugn what had already been said by Van Dams or other persons who DID testify...
Apparently the witnesses in question were not very clearly in those categories, and the judge quickly wrapped it up, before perhaps it could be determined whether the witnesses proposed would or would not, fit one of those two.