Thanks for your remarks, protect_class is the correct tag, protection_class not and should be corrected.
I also agree with you that social-protected-areas are not equal to nature reserves.
So in the styles an exception should be made for protect_class=21-29
I can adapt the code like this, I have to test if it works:
boundary=protected_area & protect_class>=21 & protect_class<=29 {delete boundary}
leisure=nature_reserve | boundary=protected_area | boundary=nature_reserve [0x16 resolution 20 continue]