You are correct – the number displayed on the game info is how many people have played the game over all time. The only indication of people actively playing the game is whether or not it’s on the Active Rail, which means at least one person is playing it on a server in your locale.
To be honest, with the way the servers work (if I understand it correctly) knowing how many people in total are playing a game at any given time wouldn’t really change much. If a game has their maximum number of players set to 4 and the game has 100 people playing it, you’d still never see more than 4 in the game. Additionally, if the game has their maximum number of players set to 20 and there are 20 people playing it, you’d get dropped into a new, empty server (which could be confusing to players).
That being said, I’m not necessarily against adding the info, I just wanted to shed some light on how those things work in case you weren’t aware