Your question seems to assume that all the people who died during the genocide were born during that time period, but I don't see why that's necessarily true. The numbers that are asserted are that the population was on the order of 20 million prior to the genocide, and 10 million afterwards (40 years later). Furthermore, the debate seems to be less over whether that many people died, than over whether the deaths were attributable to the Belgian administration.
The relevant Wikipedia section says:
[recent and older (1919) investigations concluded that] roughly half the population perished during the Free State period. Hochshild points out that since the first official census by the Belgian authorities in 1924 put the population at about 10 million, these various approaches suggest a rough estimate of a population decline by 10 million.
From p. 233 of King Leopold's Ghost Google books link:
An official Belgian government commission in 1919 estimated that from the time Stanley began laying the foundation of Leopold's state, the population of the territory had "been reduced by half." Major Charles C. Liebrechts, a top executive of the Congo state administration for most of its existence, arrived at the same estimate in 1920. The most authoritative judgment today comes from Jan Vansina, professor emeritus of history and anthropology and perhaps the greatest living ethnographer of Congo basin peoples. He bases his calculations on "innumerable local sources from different areas: priests noticing their flocks were shrinking, oral traditions, genealogies, and much more." His estimate is the same: between 1880 and 1920, the population of the Congo was cut "by at least a half."
Half of what? Only in the 1920s were the first attempts made at a territory-wide census. In 1924 the population was reckoned at ten million, a figure confirmed by later counts. This would mean, according to the estimates, that during the Leopold period and its immediate aftermath the population of the territory dropped by approximately ten million people.
Following up the citations from Wikipedia that are critical of these numbers: one (Sophie Mignon, ""Non, Léopold II n'est pas un génocidaire!") is secondary, quoting Jean Stengers, "Critique de Livre de Hochschild":
Those who spoke, in the aftermath of World War I, of the terrible depopulation of the Congo, attributed it primarily to epidemics, often appalling, especially of sleeping sickness and smallpox (...), and to the drop in fecundity caused by these sicknesses and also — these were often missionaries who spoke — to "immorality". Hochschild does not deny these factors, but in his eyes, sickness and the drop in fecundity are largely explained by the weakening of the population caused by the regime's crimes. The main point therefore stems from these crimes.
Stenger goes on to argue that the atrocities cited by Hochschild occurred at different times and places than the enormous drop in population (e.g., Heart of Darkness, on which Hochschild draws, is set in 1890, before forced rubber harvesting began ...). But he doesn't appear to dispute the drop in population size.
Ceux qui parlaient, au lendemain de la Première Guerre, de la terrible dépopulation du Congo, l'attribuaient en ordre principal aux épidémies souvent épouvantables, surtout de maladie du sommeil et de variole (...), et à la baisse de la natalité due à des maladies et aussi - ce sont souvent des missionnaires qui parlent - à l'« immoralité ». Hochschild ne nie pas ces facteurs, mais a ses yeux, maladies et chute de la natalité s'expliquent largement par l'affaiblissement de la population du aux crimes du régime. L'essentiel remonte donc a ces
crimes.
