Any reference that old is outdated. Also, not everyone who writes a canning book knows about food safety. Stick with USDA approved recipes and methods if you want to be safe. The Ball Blue Book also sticks with safe methods. There are some good online sites, too from universities that deal with food safety. I use them a lot.
I know Sure jell says that, but we tell recommend water bath processing jams and jellies. If you invert the jars, the jelly can leak on the seal. They may seem to seal, but come unsealed. Also, the seal is not as strong and can come unsealed because of it.
You won't get botulism or anything like that from the inversion method, but they can mold or ferment. Then you lose your jelly. You can't just scrape off the mold and eat it since the mold you see on top is only a part of it
growing in there. Molds on jams have been shown to have cancer producing properties.
I work at my local county extension office, and I know there have been "experts" in the field of food science trying to get Sure jell to change the inversion method in their recipes.
