It's really bad, but it's on the property taxes only. So 20% on the entire rent might still be more than required to cover just the taxes.

For math, the apartment I own and live in rents for around $2700. A 20% increase in the rent is about $6500 in extra yearly revenue. The increase in property taxes work out to be around $3000 increase.

So, the landlord made a strategic mistake by giving this as the reason. Because you can now look up the numbers yourself on the JC property website and get back to them saying that the property tax increase is only so-and-so. You'll get a bit of bargaining power.