It's actually true and have been studied and tested. I still free dive from time to time, but used to often about 8 years ago and I'm actually still a certified intermediate free-diver. My longest static breath hold is just under 5 minutes and I have completed dives over 85'. Although that may sound good it was actually below the class average. If you do several sessions of long breath holds followed by some breath up's (hyperventilation breathing) you will actually increase your red blood cells. I would do these based on a diving table daily to prepare for a dive trip.
Here is some info I found for reference:
Increased erythropoietin concentration after repeated apneas in hum... - PubMed - NCBI
Hypoxia-induced increases in red blood cell production have been found in both altitude-adapted populations and acclimatized lowlanders. This process is mediated by erythropoietin (EPO) released mainly by the hypoxic kidney. We have previously observed high hemoglobin concentrations in elite breath-hold divers and our aim was to investigate whether apnea-induced hypoxia could increase EPO concentration. Ten healthy volunteers performed 15 maximal duration apneas, divided into three series of five apneas, each series separated by 10 min of rest. Apneas within series were separated by 2 min and preceded by 1 min of hyperventilation to increase apnea duration and arterial oxygen desaturation. When EPO concentration after serial apneas was compared to baseline values, an average maximum increase of 24% was found (P < 0.01). No changes in EPO concentration were observed during a control day without apnea, eliminating possible effects of a diurnal rhythm or blood loss. We therefore conclude that serial apneas increase circulating EPO concentration in humans.
[h=1]
Breath Holding to maximise sports performance by Patrick McKeown [/h]
[h=1]
Increased erythropoietin concentration after repeated apneas in humans.[/h]
Increased erythropoietin concentration after repeated apneas in humans. - ResearchGate