I don't know why the manufacture told you that bit of information, you can buff LPU and it's done all the time. The key is to work up to the grit necessary, without removing too much material. Typically you need to go to about 1,200 or 1,500 grit before it worth considering a buffer. You can cut down the amount of sanding necessary if you use a cutter compound on your buffer, then switch to polish.
If you intend to apply paint over these areas, don't use compounds, polishes or waxes at all, they'll contaminate the surface and the paint will fish eye like crazy.
If you intend to paint over the area, then no more then 300 grit or you'll not have enough "tooth" to hold the paint .
I've "fixed" many a LPU paint job with a buffer and DuPont products. It's kind of surprising I've been doing it wrong all these years. Maybe it's because they want you to buy more paint and not cutting or polishing products.