Its an alcohol extraction ran through coffee filter at -20 degrees. Ive never heard anyone say they have had stronger oil. I think 90% is a pretty good guess.
Without analysis, it is only a guess.
When you give the drug content as 1000 mg, you are implying four significant figures. That equates to a tolerance of plus-minus 0.1%. When I taught, I hammered my students for sloppy sig figs. You would not have liked my lab course.
Back when I had access, I tested a sample of very pure fresh decarbed extract by gravimetric analysis. (Weigh the input extract on a precise balance, perform a column chromatography with each fraction validated by tlc, evaporate the pure fractions, weigh the water-clear THC using the same balance.)
My result was 89% drug content.
The commercial labs use GC or LC (high-resolution gas or liquid chromatography with spiffy UV detectors) which gets you a third sig fig.
Bottom line is: guesswork is just that. You’ve not even got two sig figs because ... guess.
That could very plausibly make for an 820-milligram brownie.
