Not likely, slander is incredibly hard to prove. You would need actual evidence that the Youtube video in question negatively impacted their business. The good news is, free speech is still highly regarded in most legal systems. There are plenty of legal protections for people who post negative reviews. Even if they are misleading/false.
@CanuckEh: First off, be very careful taking legal advice online. Everyone is an expert online. Too the uneducated like myself, this seems like scare tactics. It's extremely easy to send cease and desist orders, it's much more of a pain to open a lawsuit against someone. In America, this would be handled in small claims. (Not that anything will come of this, because it won't)
In small claims, no lawyers are allowed. This would mean it would be you versus a representative of the company. Most judge's are not going to find you liable for slander based solely on a Youtube review. Not that I'm aware of anyway.