OP, you are absolutely right to question that video - however this is a UK forum and I'm guessing you're in the USA.
I believe that in your part of the world, alarms are wired via an external transformer, making it a little safer than over
here, however, this may very well not be the case for much older alarms.
We always advise a security professional or if you just want it removed, at least a qualified electrician.
Just for others, for clarity, yes, this is the installer showing you how to disable by opening the case etc...
http://www.protection1.com/home-security-systems/support/technical/how-to/ademco/turn-off/