Group delay plot of a Moving average filter
You could have added to your question why you doubt the correctness of those numbers. Anyway, the group delay of a moving average is indeed constant. It's just the parameter 'window size' that is off by one. The window size $N$ is the number of taps of the moving average filter, and the group delay is related to the window size by
$$\tau_g=\frac{N-1}{2}$$
However, in your plot, the group delay is half of the window size, so I expect that what is called 'window size' actually equals $N-1$, i.e., the number of taps minus $1$.