Prof told me Grignards don't add to alkyl halides
Grignards can react with alkyl halides to form the expected coupling product. However yields are usually very low except under special conditions. Improved yields (but yields that would still be considered poor for a synthetic route) result when the halide involved is a good leaving group (I ~ Br > Cl), or when the Grignard is made from a stabilized "R" group such as allyl or benzyl.
Here is an interesting example where the reaction of an aryl Grignard reagent with an alkyl halide was examined.
"When a THF solution of p-tolylmagnesium bromide 1a and an equimolar amount of 1-iodoheptane under nitrogen was stirred for 48 h at room temperature, only a very small amount (7% by GC) of the expected coupling product, 1-p-tolylheptane, was formed."
So in principle, Grignard reagents are capable of reactions that look like $\ce{S_{N}2}$ reactions (who knows what the mechanism is when a metal is involved), the yields are just very low.