The problem is that it's impossible to prove a negative.
They did prove mathematically that they had a satellite signal from an area that could have been only over the ocean. They verified the math and the signal method by applying it to other flights.
Following on, they verified that they could a detect a black box pinger in an expected area based on the satellite signals and the math, and verified that several times.
They could pull up the entire plane from the sea floor today and then they couldn't prove that it wasn't planted, or teleported by aliens, or fell through a wormhole, or was sent there by Druid magic.
Because you can't prove a negative.
But you can prove signals exist or have existed and perform valid analysis on them.
When corroborated, and it has been corroborated, it's evidence. It's strength will come out, evidence is merely evidence - but it's still evidence.
This is the same as trusting GPS. Functionally, the steps are not materially different.
And that's the basis for expecting the plane to not only be underwater, but somewhere in the general search vicinity.