«مَا تَحْتَ أَدِيمِ السَّمَاءِ كِتَابٌ أَصَحُّ مِنْ كِتَابِ مُسْلِمٍ»
There is no book beneath the sky more authentic than Muslim's collection.
The transmitted statement does not require the conclusion that al-Nasaʾi judged Muslim superior to al-Bukhari. Denying that another work is *more* authentic does not logically deny that another could be *equally* authentic.
The comparative construction «أصحّ من» expresses greater authenticity. “Nothing is better than A” does not, on its own, establish “A is better than B”; equality remains possible. This distinction prevents a faulty inference from al-Nasaʾi's wording.