That is true, but after being rendered once it could be cached locally in some other way if that is believed lead to better performance.
Why do you say that? All of the metadata is at the beginning, followed by sequential frames.
The frame data is incomplete with no keyframes - if you want to do a fairly typical memory conservation technique where after user switched away from the browser tab, you pause a GIF, unload it from memory, and then need to reload it from the same frame when user switched back - you will need to replay entire GIF to that frame.