I think much of the confusion about "technical rating" stems from that word, "technical".
This is only my personal opinion, of course, but I never envisioned that rating to be something
objective, something that can be measured with hard numbers and mathematically proven. In that sense the word "technical" gives a very misleading impression.
I would say that "technical rating" is as subjective as "entertainment rating". I don't see any problem in it being someone's completely subjective opinion. Absolute objectivity is not required. If you personally
feel that the TAS is technically impressive, by whatever qualities you think are relevant, you are fully entitled to your opinion and your rating. Don't be fooled by that, should I say, technical-sounding word "technical".
Another common misconception is that "technical quality" refers solely to how frame-perfect the TAS is. In other words, if the run can most probably not be improved anymore, or by any significant amount, then it should get a perfect technical score.
I completely disagree with that. I don't think technical rating should measure frame perfection (and not only because it's something that can never be proven exactly). It can be
part of it, but only a small part.
As the guidelines say, in the same way as not all games lend themselves for a prefect entertainment rating, no matter what the TASer does, likewise not all games lend themselves for a perfect technical rating, no matter what the TASer does. The TAS could be frame-perfect, and still deserve a 3 in technical quality, for the mere reason that the game is so simple and straightforward that it just doesn't allow the TASer to show any technical prowess in its making.
While entertainment rating ought to be how entertained you were by watching the run, technical rating ought to be a (somewhat subjective) measure of how much work and effort was put into making it. And this is not measured (solely) by the number of hours put into it, but by the amount of other kind of work. With some games a lot more work like this is just not possible, while with others entire new technologies have been developed to make a faster run.
The recent
NES Arkanoid warpless run is a perfect example of a TAS that deserves a very high technical rating. The author developed an entire simulator in order to be able to brute-force an optimal path programmatically.
This is the kind of hard work I'm talking about. It's not just the perfection of the end result, but the path there, the technical work that had to be put in order to achieve this. The impressiveness of that work.
Of course you are free to interpret "technical rating" as you wish, but this is how I would see it.