Guage question

The pattern I’m about to begin states the following:

For this pattern, I’d suggest starting your swatch on a size larger than the ball band suggests for your yarn. (So, if the ball band on your yarn suggests size 8, you should work your swatch with size 9) This is because we want the finished garment to be a bit drapey.

I am using two yarns knit together, one calls for size needles 8-10 and the other for size 9. The pattern, however, is to be knot on size 11 needles. Why wouldn’t I knit the guage swatch on size 11 needles?

If I were you, I would just do a guage swatch with the 11’s. If you overshoot, then guage with 10’s; if you undershoot, then guage with 13’s.