Williamson ether synthesis requires an attack of an alkoxide on an alkyl halide. Tert-butyl bromide is a tertiary halide, which is too sterically hindered for , so it undergoes E2 elimination to give isobutylene instead of forming the ether.
Sodium methoxide is indeed a strong nucleophile (and base), but failure to form the ether is due to steric hindrance at the tertiary carbon, not due to insufficient nucleophilicity.
Final Answer: Both assertion and reason are true but reason is not a correct explanation of the assertion.