ولولا أنه تخصص لما صح وقوعه مبتدأ
Were it not made sufficiently determinate, it would not be acceptable in the position of mubtadaʾ.
The argument is that the fronted indefinite in «رجل جاءني» requires some specification to justify its being an initial subject. The reply distinguishes the grammatical licensing of an indefinite subject from rhetorical restriction that excludes other possible agents.
Consequently, ‘the indefinite can be a mubtadaʾ’ does not automatically establish ‘only one man came and nobody else did’.