I'll go further... While the plugins are not compatible, their function most likely is.
Most of the effects people use in FCP projects are Apple's built in ones. Flip, 3-Way Color Corrector, De-Interlace, etc. They all have counterparts in FCX. All Apple needed to do was translate the name and convert any values. The interface in FCX's Color Board is different, but the function is the same as the 3 Way, for instance.
Not only that, but Apple is best suited to this translation, as they know the internals of both programs!
This is not the kind of thing best left to 3rd party developers and during the years Apple spent developing FCX, they could have assigned an engineer to write this translation, so I have to think they made a conscious decision not to. They had to have decided that the number of people they would anger and alienate were not worth that engineer's time.
I believe they were wrong.