Honing is a sharpening and/or smoothing and/or grinding process. "Sharpen" is not always properly used as a synonym for "hone." "Hone in" is truly a nonsensical phrase, since one hones, but does not hone in. Think of substituting the verb "perfect" and you'll see that "perfect in" is ridiculous. You can hone skills just as you can perfect them, but you can never perfect in on them. "Home in" is an absolute must in every usage of this sort.