I'm not a doctor so I can't tell you that. All I know is when I asked my doctor about what the nutritionist told me, my doctor told me that bacterial overgrowth is something totally different from IBS and the theory that IBS is caused by bacteria is not true.
My two cents: If your doctor diagnosed you with SIBO then focus on treating that first with his/her guidance. If it doesn't go away then look at it from an IBS standpoint. I really don't know what else to tell you. I'd talk to your doctor about this instead of all of us here.