This has been a perpetual myth in a lot of fields. You see this circulating in writing circles a lot as well. In fact, I'm a little shocked that a trade magazine would purport this as accurate.
This does not prove that you own the concept. No patent, no trademark, no registration anywhere means no proof.
You have no legal ground to stand on. Chances are any civil suits launched from a sealed envelope being your only "proof" of ownership would not only fail but cost you a significant amount of money.
Any decent lawyer is going to argue, at the very least, concurrent concepts. Think of how many times you've seen the same movie come from two different studios around the same time and it's a rush to market. The reason is because we all think fairly similarly. We're influenced by the same things and can draw similar conclusions based off our experiences. All they need to do is prove they had a similar idea prior to being approached by you.
You are an independent designer. They are a manufacturer with development and research departments. How do you know that a very similar design hasn't been in a folder somewhere for a decade in one of their filing cabinets?
Any manufacturer big enough to significantly profit off your idea is more inclined to just purchase it outright. The cost of buying your idea is much cheaper than a lawsuit and the ill press that follows.