إِنَّ هَذَا التَّقْرِيرَ صَحِيحٌ.
This report is indeed sound.
إِنَّ هَذَا التَّقْرِيرَ لَصَحِيحٌ.
This report is assuredly sound.
The emphatic lām makes the second assertion stronger, but emphasis is not evidence. The more forceful expression may suit a hesitant audience; it does not, by itself, answer someone asking how the claim is established. In that situation, the speaker needs facts or an account of the method.
Conformity to the communicative setting is not simply a matter of increasing intensity. It is also a matter of supplying the kind of response actually demanded. The following description of reviewing records thus performs a role that an emphatic particle alone cannot perform.