It does also say "International Driving Permit is required unless driver has a license issued within the EU.", they then went on and asserted specifically about Indian drivers license and might have used the wrong wording (they could have used "must" instead of "should"), either way the text clearly indicates that an IDP will be needed.

Generally when it comes to IDP, it seems that each rental agency in each country have a different policy, irrespective of government agreements.