The station itself may be the deepest, but it isn't the busiest, and if you look at a map it makes sense. It's a single stop away from T-Centralan where all of the lines converge and it is the last station on the line. Leaving the station you emerge in a garden near the centre of the city, but there are other lines that will probably get most closer.
I went there in the middle of summer and honestly, the entire city didn't feel that busy in the same way London feels. I wouldn't be surprised if many locals are holidaying elsewhere, and Stockholm while a capital city, isn't at the top of the list of city destinations.
Also, shameless self-promotion and photos, but a friend challenged me to blog my trip and post online, click here for photos: https://consto.uk/2018/07/16/interrailing-into-stockholm-day...