Easy. Simple pulmonary physiology ( and some basic mathematics).
While it is theoretically possible to on-gas some nitrogen in circumstances where a free diver repeatedly descends to depth, after replenishing the gas supply in his / her lungs after surfacing between each dive, in free diving you are not - during a dive - replacing the gas in the lungs with air containing nitrogen. (I certainly didn't get the impression from the OPs comments that such was the case.). At worst, even if you on-gassed every molecule of nitrogen contained in the air in your lungs at the start of the dive (which, of course, does not happen), that amount of nitrogen is trivial - on average, total lung capacity is ~5-6 L (, e.g. only 0.18 to 0.21 CF). Even adjusting for the fact that experienced free divers expand their TLC through breathing techniques, the amount of nitrogen contained in that volume of air is, again, trivial. However, I will concede the theoretical point - if the OP made 30 descents to 150 feet over the course of an hour, then there is certainly the possibility of on-gassing a minimal amount of nitrogen. But, compared to breathing compressed air, at depth, where there is 'new' nitrogen being introduced with each breath (in fact a lot of new nitrogen at depth, because of the compression of the gas and the consequent higher partial pressure of nitrogen), which can be on-gassed, that is not the case in free diving. All that happens during a free dive - all that can happen - is that you on-gas a portion of what is in your lungs at the surface - a very (very, very) small amount consistent with the limited lung capacity of the average adult (or even the average free diver).
And, since you cite it, read the article.
"There are anecdotal and retrospective data in the literature that are consistent with a diagnosis of DCS. A recent attempt to model the risk found that it was negligible for dives to depths of less than 330 feet, then rising as a function of exposure depth until the depth was sufficient for airway collapse to limit gas uptake from the lung, possibly in the range of 755 feet.2 The magnitude of the hazard is unclear, but the absolute risk is probably very low for most freedivers, particularly when conservative surface times between dives are maintained."