I have used the machine to sew in zippers but never join 2 knitted pieces by machine. I would think they would stretch out of shape and not look good. With the zipper you have the backing to keep it flat but I find you still have to ease the knitting.

I don't think that is what they meant by sewing together. I think they meant sew together like you regularly sew knitting. The mattress stitch will give you a better seam

Like oilpainter, I have used a sewing machine to sew zippers in, but not to join knitted pieces. Using a straight stitch on a sewing machine would not have enough give and I think the seam would break very quickly during wear of an item. Knits naturally stretch, but sewing thread, not so much. Even using a stretch stitch wouldn't have enough give, in my opinion. I agree that the mattress stitch would probably be best.