You're trying a bit too hard at this point to continue defending an indefensible point of view.
The company is scummy as hell, and has both terrible staff and customer support - I'd know, I knew most of them semi-personally, and I only liked a single staff member. I've wrote them a gigantic and terrible review here on Steam. I support them exactly ZERO, after the shenanigans I've seen them pull... and yet, I'm not biased to the point I can't acknowledge they didn't do anything wrong here. They could've forgotten or purposely omitted to state the changes. They didn't. That player 6 years ago neglected to read the information available to him, jumped to conclusions and made assumptions and got burned by his own fault.
Plenty of others didn't. Because they read.

Why? That seems nitpicky as heck. They have announcements for a reason where they detail events, updates and items. You're basically stating they should spoon-feed everything to players too lazy to do minimal work.
Basically you're not even disagreeing anymore with the player being entitled to a compensation because he was right and the company was wrong, you've switched to arguing he's entitled a compensation because he was too lazy and the company is somehow obliged to state the changes everywhere to high detail for people who don't want to do minimal, effortless research.
What.
"Obscure place easily missed"?
The. Literal. Website. Announcements.
Steam. DMO. Announcements.

Those changes should be stated in game, you know, the place where EVERYONE is going to read changes and additions to the game? It's fine if they have these informations on their site or on social media. But not everyone checks those platforms for in game news. And they know that, should they should also accordingly give that information in game. If you hover over a shop item before buying it, it will not tell you if you can trade that item with other people. It will only tell you that after you buy it and the item is already in the item shop warehouse, you know, when it's already too late to know that information. Companies need to give information like that in a place they know their player base are going to most definetely read it, not in a obscure place easily missed.

The company did state these changes. I'm not sure how else to highlight this. I posted their announcement in the very same thread you commented originally quoting me.
There's zero entitlement to the refund in this occasion because they did in fact state these specifications in their announcement. It's not their fault the player in question didn't read.

That is not the same at all. It's different if the product brands itself as a mystery flavor and you get a flavor you don't like. But if a company changes a product and doesn't tell anyone about those changes and you don't like those changes, or even worse, lies about those changes, then you most definitely ARE entitled to a refund. The item shop items DO NOT tell you if they are tradable or not until you have them in your inventory, that's a blatant lack of information in order to trick unsuspecting players.

And, btw, to add to the below they actually clearly specified in the update it wasn't trade-able . The player you're defending merely didn't read it.