They are decidedly not. They are marketed in inches of the diagonal (a supremely brain-dead decision, if you ask me), but they are engineered, manufactured, and even programmed only in SI units.

If you look at your display's EDID output, the diagonal doesn't even factor in; what you do have are vertical, horizontal, and per-pixel dimensions; all in millimetres. This is what all panel manufacturers (LG, AUO, Samsung, Innolux, BOE, TCL, and so on) do.

In Germany you have to always mention the metric size. Luckily.