Unpopular Opinion

(shortened your quotes, message was too long)

IQ tests can be studied for. [...]

As far as I know, for properly administrated tests the scientific consensus is that this is not true, with a few small sample size studies showing some improvement and a lot of larger studies repeatedly failing to show anything. You provide no evidence, which in this particular case (something that goes against most of IQ research) would be warranted imo, so I can only guess what you mean in particular. As far as I know, even with potentially neglected children from environments with not enough stimulation, where the theoretical potential clearly exists, the results have been mixed.

(and it’s utterly ludicrous, incidentally, to conceive of “intelligence” as a single thing that can be boiled down into a single number!)

I don't think this is a popular claim in psychometrics and I haven't said so either.

IQ tests have major cultural components. [...].

A lot of effort went into mitigating this issue, and while it cannot be erased, it doesn't really invalidate IQ as a concept in any way. It is one of the reasons why we don't call people from more significantly different societies that one might very crudely describe as "primitive" unintelligent (and yes, some meanings of IQ can lose relevance in such societies), but afaik available evidence shows that there's not much difference between the results and usefulness of IQ in the US, Germany or China.

IQ test results vary by the quality of education available. [...]

Without more information you cannot say whether it indicated what you say or whether it indicates that more intelligent people tend to be more successful, which creates generational wealth/education differences on its own.

I am not claiming either, but let me give you a counter anecdote: Czechia doesn't really have bad neighborhoods and terrible schooling, we were forced to all be equally poor during 40 years of communism, which has only been changing quite slowly - there are about 2 real "ghettos" in the whole country, it's safe everywhere and schools are paid from state tax money, wages set by law etc. So there's almost no difference in funding between a school in a poor area and a "rich" area (with significant quotation marks), and most schools are on a similar level of quality.

Despite that, the studied qualities of IQ still apply here, and have done so since IQ research started here, even during communism where the societal differences were even smaller outside of the ruling class.

The obvious exception: you're too poor to provide proper nutrition to your children, you for live under constant existential stress etc. These likely lower your IQ and likely contribute the Flynn effect (see below).

IQ numbers have been rising over time to the point that someone who got an IQ score of 140 in the 1970s would score as a borderline idiot today.

This is incorrect and all it would take to know that is opening the wikipedia page on Flynn effect. Since different tests measure different types of intelligence and are standardized individually, it's not easily possible to say "IQ xxx in 1970 would be IQ yyy in 2020". But it seems to change by about 3 points per decade, the change has been slowing down and in some cases even reversing in some developed countries in recent decades, and the change has always been the most prominent in the lower end of the scale and not very visible in the high end.

Based on that we can be reasonably sure that a person with an IQ score of 140 in the 1970s would still be considered gifted at the least, and it is possible that they would score around 140 today as well. I'm sure that with some effort you could find some mathematician or physicist who was measured around that score in the 1970s and is still considered obviously briliant.

(if anything it might be a negative trend, given American politics in particular!). Yet if there wasn’t a built-in corrective factor applied that changes each year IQ scores would be rocketing skyward. Again this hints at something learned, and not intrinsic.

See above. I'm too lazy to go find if the US suffers from reverse Flynn effect, but there have been researchers claiming that median IQ has been going down, though I think it's not a mainstream consensus opinion. In any case, IQ has not been skyrocketing in the US for some time as far as I know.

Furthermore, the Flynn effect is an effect widely studied by actual scientists, it's not a thing that disproves psychometrics, it's an area of research of psychometrics.

So IQ measures something … but nobody can say what it is.

Literal books have been written on this. You just have to read them. The IQ is used because we know that it's a useful metric for many things, it's pretty much as simple as that.