Well, no your not really wrong. There is no such thing as a "PC" harddrive or an "Amiga" harddrive or a "MAC" harddrive or a "Linux" harddrive for that matter.
A harddrive is a harddrive. You buy it from the manufacturer IE. from Conner, or Western Digital, or Maxtor, or Seagate, or whomever, you don't buy it from "PC" or "Amiga" or "Microsoft" as they don't MAKE harddrives.
Maybe what he meant though was since this is a 3.5" inch harddrive it won't fit inside a standard A1200 with out an external case or some modification and/or 2.5 inch to 3.5 ide adapter etc. A1200s are desigened to take standard 2.5" IDE/ATA harddrives or course. So maybe that was what he was talking about??