Pixels likely refers to 1x1 tracking images (image fetched from a server, and through that, they can get some data from you, like IP address or visited before)
Sorry, I sent my comment before it was done. It's edited now.
Yeah, I see your edit. With JS you can also detect whether it was fetched before, so that is a way of storing information about whether you have visited something before. Which can then be sent to the server. So, it kinda is storing data, kinda not.