You either been had or a victim of the seller's mistake.
You touched on the obvious stuff: the wire gage, the volts rating, etc but there's other factors making a cord assembly suited for use in an industrial setting, crush and oil resistance, lugs and connectors, testing certs, approvals, etc.I refer you to the NEC for the details. Chances are the cord you have will work for now but why take a chance with fire, employee safety, work stoppage. A 100 foot cord would normally be selected a wire gage larger than the code required ampacity for reasons of voltage drop.but the effect of voltage drop over 460 volts is a matter of proportion - ie, to lower voltages so that's a judgement call.
In your case, a commercial shop constrained by rules and reg and concern for employee safety. If you can't get your money back, I'd suggest you replace the plug and connector with those intended for lower voltage and send Mario, the knee-capper, to explain electrical theory to the seller. In any case, pony up the $300 (probably more) for a properly rated cord set for your application..
Making a 460 volt, 3 phase, 30 Amp rated extension cord aint rocket science. It's a first year electrician apprentice job. Your components have to be properly rated for the specific service and, of course, the workmanship and other details up to snuff. I suggest 600 volt rated rubber covered 8/4 cable having a SOOW or better rating and service specific terminations by Hubbel;, Levitron etc. But look in the NEC. Such a 100 ft cord at 65# will be all anyone would want to carry any distance.
While I'v been pecking away, others have responded quicker.