That is complete bollocks. (sorry)

I (until very recently) had a 2.9 V6 Granada Estate in my collection. 2 x Transponder immobilisers cutting different circuits, including cutting the ECU ignition feed where it passes behind the passenger windscreen vent (impossible to get to unless you remove the dash), plus a Toad alarm system on it. Someone had obviously been watching it as the mother-in-law uses (used to) it for work and is in the same area every day. They smashed the window, took the handbrake off and selected neutral on the auto box and dragged it onto a low-loader. I know this as there was still a black skidmark from the drivers' rear wheel as the caliper was sticking and used to have to free itself off. Car is almost certainly broken for bits now.

Lesson: a car alarm / immobiliser will only stop the opportunist or amateur car thief. A determined or well-prepared thief will find a way to remove it.

If you want a good way to stop someone driving off with your car, either cut the ECU ignition feed and hide the supply or remove the ECU. @ least that way, if they manage to drive it away, you know it is someone who knows the car and know where to start looking.