The claim under criticism is that a nuclear bomb in space could turn all its released energy into a laser beam. That degree of conversion is rejected as beyond present materials capabilities. A much later breakthrough is left possible. Fictional descriptions of such a weapon do not establish that the engineering works.
Notable Point · 1:36:07-1:36:52
Bomb-pumped lasers and the gap between idea and weapon
A claim that a nuclear explosion could transfer all its energy into a laser beam meets a materials objection.