Impolite and "toxic customer"/"Thief" are worlds apart. If you can't deal with a broad selection of the average public, don't sell to them. It's really that simple, and the business world selects heavily against people who think that their product or their fiefdom is excepted form that rule.
It's not about the lines that I draw or that anyone draws, it's about the fundamental power asymmetry in customer-business relationships. Obviously a bar or a B&B have to consider the whole pool of customers, but if you're in the business of selling someone an IoT garage door opener...
I admit that others view it differently. But I'm not apologetic about the idea that a business can choose how much rude behavior to tolerate. Especially if I'm willing to refund without return of the product.
Your call, I suppose it's subjective. But I personally consider it to be rude. Would it be an appropriate support response to suggest that maybe their phone is a piece of shit?
It's only rude if you believe that the customer has to respect the hard work and good intentions that went into the product, and make allowances if it doesn't "just work". They don't.
Similarly, I don't have to make allowances for people that can't manage basic civility. I wouldn't send back food with a "this is shit" comment at a restaurant. Even a fast food place. Because I'm interacting with a human being, and see no reason to be a jerk. As a side bonus, "sorry, this tastes bad, could I get something else?" has a higher likelihood of resolving the problem.
That appears to be before any contact with anyone at all.
That, alone, doesn't cross a line. But it is a clue that the interaction might quickly lead down a path where you choose to part ways.
I'd have been initially polite, helpful, try to fix the issue, etc. But there would be some line where I'd stop.