Hello,
Actually, on your Site the Additional Information tab was the default one which is why our applied CSS wasn’t working that way completely because when the Product page is getting loaded, that Add. Info. tab was getting loaded first and that’s why the Tab Hiding CSS is only working for Tab Title.
We have now added the Description for a Product on your Site on which we were checking and then checked back the concern again, our CSS was working fine by then because the Description is now the default one: https://snipboard.io/2OJdvm.jpg
So, you need to have Description for the Products and the CSS for hiding Add. Info. tab will automatically work.
Please check back your Site after completely clearing browser and server cache.
Best Regards.