Here's why I assume aluminum:

If it was plastic, they would have said plastic. Everybody knows what a plastic water bottle is and that's how they talk about it. Instead, they used the word "disposable."

Why use "disposable" when plastic is more clear? Nobody calls a plastic water bottle a disposable water bottle. They are probably using a curious word choice to try to downplay the fact it was a metal bottle.

And Connor Pay said it was aluminum.