It looks worse than that. In figure 1 of the paper[1] they only make a comparison between Bajau and Saluan populations. They show the "enlarged spleens" have volume of ~200 cm^3.

However, if you look at humans overall, apparently even these "enlarged spleens" are much smaller than the average spleen:

>"The mean splenic dimensions were ... 333.6 ± 116.1 cm2 in volume." https://www.ajronline.org/doi/abs/10.2214/ajr.184.1.01840045

[1] http://www.cell.com/cell/fulltext/S0092-8674%2818%2930386-6

EDIT:

They write:

>"We made ultrasound measurements in two planes such that we were able to calculate spleen volumes according to the methodology outlined in Yetter et al. (2003)"[1]

So lets look at Yetter 2003 for their spleen volumes, in table 2 it shows they are >500 cm^3 on average: https://www.ajronline.org/doi/abs/10.2214/ajr.181.6.1811615

So it looks like these "super spleen divers" actually have relatively small spleens...