im pretty sure its per coil....
i've used a very silimar 3hp based woofer that took an mtx 1501d @ 1ohm daily no problem, and burped .5ohm easily as well. at least 1500w rms daily and it was a champ. The coil will outlast the rest of the woofer...the 4hp ti pro's sometimes had the issue of the tensile leads failing, thilo in turn doubled up on those leads to help deal with the current demands on them. I've persoanlly heard from thilos mouth that the 4hp ti pro is rated 1kw rms per coil, and from my personal experience abusing TC sounds woofers im confident these will take a clean 1500w each all day.
it's not that unreasonable for a website like sonic electronics to get a detail wrong, these things can happen when a manufacture rates per coil while most other do it per driver.
that just my $0.02