Why would it be irrelevant? That phrase it the argument for it reflecting, which was one of the conditions used to prove it as light speed in the first place.
The sound point is relevant, because if it made sound that means it had force, which means it has mass
Even accepting the phrase as "turn aside" does not really help
Definition of turn aside in the Idioms Dictionary by The Free Dictionary
idioms.thefreedictionary.com
"To deflect something; to direct or divert something away"
reflecting something and deflecting something are different
Reflection is when something (light, heat, etc.) bounces off a surface without being absorbed
Deflection is just changing the direction of something
I can deflect a tennis ball, for example, but no one would call that reflection
The beam would have remained continuous and been reflected toward a different target, which it would burn
In this case, it was deflected by the sword, but stopped there
It didn't reflect toward a different destination the way that real light would have