Light travels through a closed cylindrical tube filled with a gas....

The speed of light in a medium is given by $v = \dfrac{c}{\mu}$, where $c$ is the speed of light in vacuum and $\mu$ is the refractive index of the medium.

The refractive index of a gas is slightly greater than $1$.

As the gas is gradually pumped out, the density of the gas decreases, causing the medium inside the tube to approach a vacuum. Consequently, the refractive index $\mu$ of the gas decreases towards $1$.

Since the refractive index $\mu$ decreases, the speed of light $v$ inside the tube increases.