Why would a Martian tsunami travel at only 200 km/hour, four times slower than on Earth?
The linked paper does numerical modelling of a hypothetical tsunami and predicts speeds between 60 m/s (at its point of origin) and 30 m/s (nearer to shore). We can try a back-of-an-envelope approximation to see if we get close, and to understand why the speed would be so much lower than on Earth.

A tsunami behaves as a gravity wave with a very long wavelength. In shallow water (and pretty much anywhere in Earth's oceans counts as shallow water for such a long wave), the speed at which it moves is approximated by

$$V = \sqrt{gD}$$

where $V$ is the wave's speed, $D$ is the depth, and $g$ is the acceleration due to gravity.

On Earth, $g = 9.81$ m/s2 and, in the deep ocean, we might assume a depth of 4 km and obtain a speed of 198 m/s, or 713 kph.

Let's try this for Mars. As you've identified, the acceleration due to gravity on Mars is about 3.8 m/s2, and the linked paper gives assumed depths of the ocean on Mars as between 100 and 1200 m (forgive me if this is not quite right - I've only skimmed the paper, and these figures come from Table 1). Let's assume a 1000 m depth, since the highest speeds will be found in deep water:

$$ V = \sqrt{ 3.8 \times 1000 } = 61.6\ \textrm{m/s} $$

which is close enough to their value for comfort. So a tsunami on the Mars described here would be much slower than on Earth due to the weaker force of gravity and the shallower depth of the ocean.