does "clear opening" on rafter loading tables mean horizontal projection?
Yes, the “clear opening” is the horizontal distance from support to support. (We often call this the clear span.)

In your case, the clear opening is about 14 feet. In fact, the clear span is the horizontal distance from FACE of support to FACE of support. It doesn’t matter the length of the member or if it’s a 4:12 pitch or a 12:12 pitch.