An initial explanation of «لَا رَيْبَ فِيهِ» is that people who remain doubtful are addressed as though they were not genuine deniers, because adequate evidence is available to them. Their hesitation is not granted the standing it outwardly claims.
A difficulty remains: the expression does not merely omit an emphatic particle from a positive report. It negates doubt comprehensively. These are not precisely the same rhetorical operation.
Accordingly, the discussion proceeds to a more exact explanation. The first proposal should be read as an interpretive stage, not as the only final account of the passage.