Not sure what your argument is exactly. Snakes with heat pits as a general rule do have vertical pupils, yes. But so do boas and as you know, they don't have heat pits.

Forget whatever trick your own eyes played on you. Rattlesnakes (including western diamondback) are pit vipers, and as such, have vertical pupils. period.

You probably saw that snake when it's pupils were quite dilated. When they are like that, it's hard to spot the vertical pupil, as in this picture, but if you look very closely, you'll see the front edge of the pupil is somewhat straight. That's a vertical pupil. The back side often looks round when they are dilated.