That's not how it works. They can write whatever they want on the packaging. It's the final assembly that's compliant. They're covering their ass in case you put their 5W bulb in some application they've never heard of where it technically fits but a 2.5W bulb was supposed to be used or something.

It's like how aftermarket brake hoses all say "off road use only" despite pretty much all of them vastly exceeding the FMVSS for brake hoses.

The correct bulb will not say that, aftermarket LEDs do. The light reflector housings are designed and tested for specific bulb standards. There are LEDs which try to output light from the same place as the filament in the bulb they are mimicking. But there is no guarantee they function properly, hence the warning and illegality.

If you swap one side and walk around your car, you may see that they are significantly dimmer than the stock bulbs from some or all angles. Or it may work fine. Often times the aftermarket LED dual intensity tail/stop lights have barely any difference between the two brightnesses which is egregiously unsafe