Diving itself might somehow enlarge the spleen. There are plenty of examples of experience changing the body, from calloused feet to bulging biceps.
Only some Bajau are full-time divers. Others, such as teachers and shopkeepers, have never dived. But they, too, had large spleens, Dr. Ilardo found. It was likely the Bajau are born that way, thanks to their genes.
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...
>"Normal cadaveric splenic volumes reported by Loftus et al. [8] are 26–250 cm3 with a mean volume of 110 cm3 and an SD of 70 cm3. Henderson et al. [19] reported a normal splenic volume of 219 cm3 as calculated from axial CT acquisitions."
So an average of 219 cm^3 according to axial CT is still larger than what is seen here. I'm not sure about the cadaver spleens, do they shrink? The Loftus paper only reports maximum length vs actual volume so we can't see that direct correlation.
Some other papers report 184 cm^3 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4549594/) and 214.6 cm^3 (https://www.ncbi.nlm.nih.gov/pubmed/9038125)
“I would think, as morbid as it is, that if they didn’t have this, it would kill them,” she said.
Yeah, and by a similar leap of logic, if we didn't have two legs, we'd flop around on the land like fish.