Amusing thread. There's so many people arguing that the ETA is not an ETA and come up with handwavium to explain the HUD, when a simple look into the manual would probably help.
"A destination that you have locked into your navigation computer is still displayed during super cruise as a canopy element with distance and ETA displayed" Page 72, top.
OP is right- the ETA timer is, if used to get an ETA estimation, useless. Period. No amount of arguing, explaining, handwaving and being-right-really will make the fact that a 7s ETA, and it is an ETA per the manual, will take far more than 7s. Full stop.
Everyone else who's arguing that it is a useful tool is of course right, too. The magic 7s to get to 75% for a magic drop in speed just to make it right timed for safe disengage is useful. It's just not an ETA.
This has been debated on this very forum ad nausem, and the exact same arguments always come up: The ETA is right that moment in time but as the ship speeds up/slows down it is changes, it can't plot an accurate ETA due to gravity wells, it doesn't know your course, etc etc. All right, all correct. None of that makes OP wrong: The ETA is no ETA.
I do what I do with most things around Elite and ignore that it's described as an ETA and just use it as an indicator when to press a button to slow down.