Right. Speed is determined by the slowest device in the path.Fiber optic cable is just (part of) the physical transmission line from your home to tie into their (whoever 'they' are) network. The hand-off to the back-end is typically the bottle neck. After that you are still dependent on their network: routers, switches, servers, etc..
The bandwidth on fiber is "dedicated" compared to cable, but read the fine print to see what they are really guaranteeing compared to what they are advertising/billing for. Cable, by comparison, is shared (the more people surfing, the slower the speeds).