in order for Company A to sue you, they would have to prove without doubt, that you hunted down their list of clients and prospective clients, with the SOLE intention of selling them the product from Company B.
That is some really bad advice.
They don't have to PROVE anything to file a suit. If the case ever get's to court they will be required to prove their claim but there is no requirement to prove their case when the suit is filed.
While it is true that many non-compete, non-piracy, non-solicitation agreements may be difficult to enforce if the one bringing the suit has deep pockets and want to use you as an example, you need to be prepared to defend your case.
It cost me $20k to prove there was nothing to the suit brought against me but I had no choice but to defend myself.