A lot of interviewers grill you on stuff that they know a lot about, because how do you interview somebody on stuff you don't know anything about? So, what winds up happening is that you have to keep interviewing until you wind up with a set of interviewers that has a lot of knowledge overlap with yours.
But, at least, you have standardised pools of coding questions, standardised pools of system design questions, and a standardised feedback form.
The interviewer varies a lot though... And makes all the difference when you have an inexperienced one, or someone that doesn't care.
I believe this is likely the main point being made about luck - be unlucky enough to hit up an inexperienced interviewer and all the time spent preparing as in the linked OP is up in smoke.
Not sure if it counts as an average tech company, but in terms of non-FAANG I think startups and smaller companies can end up relying less on luck. There is more time and motivation to truly flesh out candidates, maybe giving non-interview coding tasks and such to balance out interviews. At a FAANG if there was an issue because of an inexperienced interviewer, again you need luck for a recruiter or committee to attempt a save among all the other applicants.