I agree: tourism=caravan_site + caravans=no sounds a bit strange, but it comes because the value caravan_site is used also for RV sites (tourism=motorhome_site does not exist, perhaps it would be better to create it, but it is another story).
Nevertheless, I stand by my position: you cannot add capacity:caravans if caravans are not allowed on this site!
... and it seems that I'm not the only one to think so.