Why couldn't he have noticed a performance degradation and suspected a poorly done de/anti icing? Maybe even substantiated by a visual check?
Or do we now have to assume the minimum-wage dude freezing his b@lls off in the nacelle spraying toxic stuff all day always does a good job?...
The shadow of a doubt is enough reason to divert. Especially before an arctic crossing in wintertime.
Safety first, remember? Or has the company drivel got you to start thinking like them?