if they are harrassing you over 1 comment it shows how much weight it must be carrying . tell the company you will join 50 similar forums and have your bots post daily comments about their dangerous product if they persist in their bullying tactics .
That's committing a legal suicide.
Please, OP don't do anything that will weaken your case.
Do not accept any kind of payment or benefit to delete/alter your comments. If you do so, the company may prove that your were using your comments as a way to blackmail them.
Your comments are absolutely fine and there is nothing defamatory about them.
You made the comments in good faith and I don't think you need prove anything.
Nevertheless, you should contact a lawyer and send a legal reply to the company.
I have faced such threats in the past and usually a counter-notice from a lawyer is enough to end the matter.
Not sure about your country, but in my country, defamation cases are a waste of time and companies don't go beyond notices.
In many countries, there is no fixed jurisdiction for defamation cases and defamation cases can be filed anywhere.
I don't know much about laws in US, but to explain my point here is an example.
You may be located in NYC. The company suing you may be located in NYC. However if the company is selling its products in Los Angeles, it can sue you in LA saying that their reputation in LA was damaged by your comments.