After you've been sold a product, the manufacturer has no authority on how you use it or where you resell it. Since the manufacturer doesn't have this authority they you can't be legally circumenvting authorized channels when reselling it.
There is nothing "gray" about second hand.
But if I buy a similar lens from the camera shop down the street, and it was originally sold in the US and the warranty is still valid in the US etc, it's a USED lens or a second-hand lens, but there's nothing gray-market about it.
I think you're spot-on here; sources quoted in the article are misusing the term gray-market to simply refer to second-hand parts, which are something completely different. They want to make pre-owned parts sound just as shady, but the're not -- the first-sale doctrine would like a word.
How do you know this? More to the point, how does an amateur buying their first lens ever know this?
My friends and I used to buy surprisingly good deal products on Craigslist and see how long it took to determine if they were fakes or broken. It was always one or the other.
I'm sure I could take a lens to them and have it checked out for a few bucks, or have them broker a sale between me and another private party. Knowing what I'm getting has its own value, and I'm not opposed to paying someone for their time and expertise.
tl;dr: I don't, personally, know that. I trust someone else to know that.
Unfortunately, this idea is not as settled as you make it sound (around the world). Even in the US, it still pops up - in 2019 SCOTUS heard a case on cheap imported college text books.
Additionally, manufacturers have latitude on who they sell to, and to sign contracts with those intermediaries.
To show how absurd this subjective projected pejorativity is, lets use it to 'second hand':
>The question is not "Does the term second hand market have a meaning?" the question posed was "Is the use of the term perpetuating an unhealthy view of commerce to advantage manufacturers?".
>There is nothing "second" about selling used products.
"gray" sounds like it's questionably legal and halfway to "black market". The negativity is definitely part of the term, not imagined.
Has something been done to it to void the warranty?
Is this a cheaply done "refurbished" version which will die in 3 months?
Is it stolen?
Seems pretty gray to me.