The negation (of recitation of the Bismillah) in the report of Muslim can be understood in light of this report (i.e. it was recited silently), as opposed to those who declared Muslim's report as being defective.

وَعَلَى هَذَا يُحْمَلُ اَلنَّفْيُ فِي رِوَايَةِ مُسْلِمٍ , خِلَافًا لِمَنْ أَعَلَّهَا.‏ [1]‏ ‏‏.‏