When the passengers can see the water, they aren't receiving conflicting information: both their eyes and their bodies experience turbulence. This affirms the correlation between conflicting information and motion sickness, supporting the hypothesis.

This doesn't tell us if planes feature the same “conflicting information” phenomenon as space shuttles, so knowing that passengers sometimes get motion sickness doesn't help us.

In other words, motion sickness can occur even in the absence of conflicting information. This undermines the hypothesis, because it suggests that something else causes motion sickness.

People with different views should have a different degree of conflicting information. This tells us that they all get motion sickness equally, which undermines the connection between conflicting information and motion sickness.

The author never claims all astronauts get motion sickness. The fact that not all astronauts get motion sickness isn't relevant to what causes motion sickness when it does occur.