Only the appliance manufacturer could give you a definitive answer how their product differs in that respect, however, I suspect it's likely just to cover themselves if there's a claim made due to 'improper' use, contrary to their warning.
Any gas appliance, natural or LPG has the potential to cause carbon monoxide poisoning and obviously, the more-confined space such appliances are used in, the greater the risk.
There's likely no greater risk with you using that appliance as you have done with others, but by using it against the manufacturer's advice, you would do so at your own risk.
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