You are correct, a GPU is not needed on the server if your models are intended to run in WebGL streaming mode. WebGL streaming became the default mode starting in FlexSim 2020 Update 2.
A model can be toggled to use the video streaming mode. If your webserver will ever stream models in this mode, you'll want to be sure that your server meets FlexSim's graphics requirements. To use video streaming, add a streammode node with a value of 0 to your model tree in MODEL:/Tools/serverinterface/streammode.
You can read more about WebGL and video streaming modes in Webserver (flexsim.com), under the 3D View heading.
There are some limitations with WebGL streaming where some modeling assets are not rendered. Please see Phil's answer here for further explanation.