I just wanted to add that this same constraint option also applies to Ctrl+drag of objects so IMHO it is reasonable to assume that rotation constraint was broken when this feature was introduced and that the constraints were conflated, confusing the developer and the users (well, me at least) in the process.

From Pro X (v8)

Ctrl+rotate alignment
When rotating objects if you hold Ctrl down it now snaps any significant straight edge to the horizontal or vertical. This applies to text lines, columns and blocks, photos, Quick Shapes, lines and even freehand drawn shapes that have significant straight edges.

From v7

Non-axis aligned snapping
You can now also align to lines that are neither vertical or horizontal.