Does he think people in the order world drink out of puddles?
...or that his grandpa's choice of kitchen supplies would change as soon as he crosses the door...
The logical conclusion would be that if his grandpa liked drinking tea, he'd simply have brought his usual teacups through the door from the kitchen on the other side. Even if isekai teacups are somehow different, why would his grandpa go shopping in iseaki-town for something as mundane as teacups when he already has some just a few steps away...
He really chose the stupidest thing to be surprised at.
...Also...
THE CAT KNOWS!
She clearly knows that the tea is awful and his grandpa, like himself, was too kind to say it.