I think the answer is in the paper:
The formulas below do NOT work with "extreme scores" (perhaps over 1250 SAT)
so it is validated over a range and not outside, so the answer to, "is the formula accurate?" is - - no
and your hidden agenda which is "this is a load of crap when it comes to assessing children for the rest of their lives" is I think absolutely true. I am not keen on measuring IQ and then drawing conclusions from it other than, "oh look it is high"
WE ( Brits, people who live in London) dont do SATs, we do A levels and they are pretty useless for assessment too.
You have to use something and give as many candidates as poss a chance. Fifty years ago this meant that they chucked out half the year at the end of first year of uni.
Newton's successor (Taylor I think) 1740 wondered why grads of Newtons uni did badly compared to the French and Germans.
1840 Babbage and Faraday were still wondering. 1950 People noticed French math grads were hitting above their weight and asked how they did it (bourbaki and grothendieck) . 2000s, a Boris effect: the best grads at Greats (oxford) and Maths ( cambridge) - just seemed to be good at doing the questions you got in Greats and Maths uni exams. ( and by implication no much else. )
I dont know what the answer is, I am not an educationalist. One thing I do know: you should be tested on a course and not something else. - (personal experience)
this answer is probably as good as it gets