That's blatantly incorrect.

The canonical example of this is "Aeroplan only let me book YVR-YYC-YWG-YOW-YYZ, and I SDC'd to YVR-YYZ". That's always permitted, and I've done things like that more times than I can count.

I have added a segment in the past, but it is difficult. One of the benefits of AC SDC (compared to, say, UA) is that they don't reticket anything. It's just a change in the AC PNR. This means they can do more interesting things, like SDC codeshares and foreign tickets, as long as it's operated by them.

However, it also means the number of coupons doesn't change. So if you have SFO-YYZ booked, you have one coupon. If you SDC to SFO-YVR-YYZ (I did that once for a pod), you end up with COUPON REQUIRED and need a FIM.

I've been told "no" more than I've been told "yes", but I've definitely done it.