They do it through fictional characters who have the knowlege to say so.
Hashirama stated that Sakura's CES could be stronger than Tsunade when his last memory of Tsunade was when she was 4 years old. Yet you still think that his statement can be used an evidence but not Shikamaru's. When Shikamru's case make far more sense than Hashirama's. Shikamaru is not the type of guy who make baseless statements so he should have some knowledge about the five Kages.
Shikamaru was only saying the people present were extremely strong and capable, he never made a direct comparison between anyone. If Sakura was standing right F*cking there, he wouldn't have been like "nah you aren't strong enough stay here".
What he stated was "Of our
current members here, these are the best as far as combat ability goes". The bold can be possibly referring to the whole village though that might not be the case but regardless, Sakura was present in the village yet they didn't even call her. If she was stronger as you claim then why didn't they want her help in saving The Hokage and the whole world from The Otsutsukis instead?
What Hashirama said was a completely different case. It was shitty writing for him to have knowledge of Tsunade's full strength, but the comparison was made extremely clear which is why it can be used as evidence. Kishimoto wasn't thinking about implying Sakura was weaker when he drew that panel. Know the difference.
Ok. But you still need to explain why doesn't Shikamaru statement prove Kurotsuchi superiority. If there was someone present who is stronger than the five Kages then why didn't they ask for his/her help instead?