To be fair, unless the tax laws / terms are significantly different in France than here (a distinct possibility) a "90% deduction" means you don't pay taxes on 90% of the money going to the cause. In the US, such charitable donations are 100% deductible (I don't think there are limits to that either). If you give $100 to such a cause, you save (depending on your tax bracket) up to $36 in taxes that you otherwise would have paid on that income (based on probably-outdated memory of the top marginal tax rate being 36%, which I know it no longer is, but humor my laziness in not looking that fact up).
Now, that having been said, maybe "réduction d'impôt" is more akin to a tax credit, which would mean that, yes, that "philanthropist" is asking the government to essentially do a very significant funds match on their donation so that they get $90 back from that $100 they donate (although presumably they would have paid taxes on it as it wouldn't be both a deduction and a credit, so really they paid say $36+10 or $46 and the government chipped in $54 on that $100 going to the charity, were we talking about US taxes).
All total, charitable deductions are tax deductible specifically to encourage people to pay taxes. There is a longstanding myth that "it's deductible" means it is essentially free, which is not the case. It just means the government is chipping in a good amount on top of what the "benefactor" is actually putting out. This is why the tax code is "so complex": it is one of the most powerful tools the government has to encourage people to act in favor of the greater good instead of just selfishly (with the presumption that most charities are "greater good" charities, Trump Foundation et al notwithstanding).