Andrew wasn't really trying to scare them or threaten them, as they didn't know each other and it wasn't like Andrew had saved them on several occasions, for his words to have any kind of weight.
He was just telling them the truth. Although he saved them with the intention of establishing a friendship that could serve them in the future, that doesn't mean he would beg for that attention.
On the other hand, the water elves misinterpreted his words as a threat, but a very weak one, since as it had already been said, they barely knew each other and it was only the first time they were saved.
Even so, the water elves did not immediately accept and ignored his words since they were still in the presence of Oliver, who could easily capture them, so they had to think very carefully about their next move.
