One of the arguments against using the term "timeattack" was that it was already widely used in many racing (and other such) games as a gameplay mode where you run against the clock (usually contrasted to racing against opponents for the first place). While the meaning could be seen as related, it's a bit too narrow in that context and could cause confusion. "Speedrun", on the other hand, was an already well-established term. ("Tool-assisted speedrun" might not have been as popular and well-established because back then it was limited to a really marginal set of speedruns, probably only in the Doom speedrunning scene, but it was established nevertheless, to mean the same as "speedrun", but using tools like slowdown and arbitrarily many segments.)
On a tangent, I have for long suggested we move away from using the term "movie" to denote the timed keypress recording file, precisely because of its potential for confusion. (To the average person there's little practical difference between "movie" and "video", and it may cause the misconception that we edit video files and consider them "tases".)
