13.5C9 means dominant 9, not major 9

This follows the same rule we learned for 7ths.

Cmaj9

C E G B D

C9

C E G B♭ D

Because plain C9 implies a dominant seventh underneath.

Formula:

1 3 5 ♭7 9

So:

plain 9 = dominant 9 maj9 = major 9