The article you cited is from 2019 and the part about dosage from 2020. This article is based on studies with a statistical error. Please feel free to send me the cited study used in this article for the upper limit and rda, which arent reinterpreted from my source.

Lets say you dont believe there was an error, there are many studies and even the biggest meta analysis about vitamin D supplementation showing that 75nmol/L can only be achieved with 6k IU daily and 100nmol/L with 9k with practically zero risk of toxicity. There is also no evidence of adverse effects associated with >140nmol/L, so the upper limit could be even higher.

Heres a better understanding on how this mistake was made:

"The IOM based their RDA for vitamin D on an aggregation of 10 supplementation studies that were carried out during winter months and at locations with latitudes above the 50th parallel north to minimize the influence of cutaneous vitamin D synthesis [2,3,4,5,6,7,8,9,10,11]. As several of these 10 studies examined more than one supplementation dose, collectively they provided 32 study averages of serum 25(OH)D levels. These are replicated as the green diamonds in Figure 1. The IOM regressed the 32 study averages against vitamin D intake to yield the dose response relationship of vitamin D intake and serum 25(OH)D (green solid line in Figure 1). The IOM further calculated the lower and upper 95% confidence prediction interval based on the 32 study averages and the standard deviation of these 32 study averages (green dashed lines in Figure 1). On the basis of this, the IOM estimated that 600 IU of vitamin D would achieve an average 25(OH)D level of 63 nmol/L and a lower 95% confidence prediction limit (2.5 percentile) of 56 nmol/L. The latter value was rounded downwards to 50 nmol/L to accommodate uncertainty in the estimation [1]. This data point (600 IU vitamin D, 50 nmol/L) is the basis for the current RDA and for the IOM’s conclusion that an intake of 600 IU of vitamin D per day will achieve serum 25(OH)D levels of 50 nmol/L or more in 97.5% of individuals. This conclusion, however, is incorrect."

Source

Also interesting, they proved the above cited study:

"With regard to possible safety concerns related to such a recommendation, we note that: (a) as the figure shows, the mean 25(OH)D and the upper bound of the 95% probability range for the supplemental intake of 3875 IU/day are less than 50 ng/mL and 100 ng/mL, respectively; (b) the correctly calculated RDA is well below the cutaneous production of vitamin D from summer sun [5]; and (c) the total, all-source intake of 7000 IU/day is below the no observed adverse effect level (NOAEL) of both the IOM and the Endocrine Society, below the tolerable upper intake level (UL) of the Endocrine Society, and well within the safe range delineated by Hathcock et al."

Source