No, not always. It does depend on people submitting trade data to it though (via a variety of third party programs), so if not many people have visited the systems in question
and they haven’t been adding the trade data, the prices can get inaccurate. There is a “prices last updated” field on the listings for a system, so you can get an idea of how current they are from that. Persephone, for example, last had a price update 7 days ago. Which suggests that the current controlling faction either aren’t paying attention, or they are being highly covert.