The French property tax is not bizarre as much as absurd, obtuse and obsolete. The index hasn't even been updated since 1970, which means that it's completely disconnected with real-life rental values. Every single addition or improvement costs you something (an equivalent of 4 sqm is added to your official surface area simply for having running water, 3 sqm per heated room, etc), though it must be noted that you can't even properly simulate the impact of any improvement. I just received mine and my flat's rental value is computed based on input data I don't have access to as being roughly one fourth of current market prices. On the flip side, I'm paying the equivalent of 43.118% of its theoretical rental value this year (on top of income tax, obviously). The system is honestly completely messed up. A reform is apparently coming in 2026, and though it seems that it will be quite fair, it's mostly an excuse to increase taxes by an another 15% on average :(
it's apparently proving very difficult to reform as there would be as many losers as winners, and the losers hold more political clout (people with property in hot markets).
Referring back to the article, from what I'm seeing pools aren't even taxed that much. In most places, you're seemingly paying some €80 per year in additional real estate taxes for a standard 4 × 10 m pool. Probably much more interesting for the tax office is that if you're cheating on something as visible yet fundamentally cheap as a pool, you're probably not being very honest about your other taxes either…