Based on the website statement it is overall within the players wins so still the casinos are the one who are paying with the tax of 10%, now if the casino makes a change so the players will not win on the game its up to them but imagine if the player discover that they arent getting fair to their players still consider them as part of the lose of their summary of earnings. Actually that's quite low instead here in the Philippines afaik they are having a 30% tax to the casino just to operate here in the country.
Zimbabwe is a developing country and poverty is very high, so it is not surprising that a large number of citizens resort to gambling. The authorities have certainly noticed this and want to benefit from it in any way possible. The tax is imposed to finance the poor budget and not to deter people from gambling as some may think.
10% does not seem like a large percentage to me as a tax on gambling profits. I am also from an African country and taxes on gambling profits can exceed 50% if the amount exceeds a certain limit in addition to a fixed law that imposes 25% taxes on all profits from bets, competitions, television games, etc.
It is not surprising that this percentage is preliminary, awaiting an increase later if satisfactory results are achieved. I doubt that any country imposes such low taxes on gambling activities.