My hts 5000 manual says no such thing, at least at first glance. In any event, that disclaimer could be for only two reasons (that I can think of);
1) an extension cord would potentially create a trip hazard, injuring the person, as well as the equipment; and,
2) voltage drop.
Regardless, that's good advice j_garcia to recommend against an extension cord run through a dropped ceiling, or any other manner for installed products. Voltage drop concern in my install is nil as I used 12awg. I think the permanently wired solution for projectors is not only novel, but money saving (no need for a second filter or conditioner). Cheers.