Yeah, Aspidistra, that’s definitely not USDA-approved. I know that’s an old, classic method, but no sense in risking botulism over some canned jam. My fought-in-WWI grandpa would do similar with raspberries but he’d freeze the ones that he wanted to keep long-term, and refrigerate a “use quickly” portion.
The method of home-canning of (most) jams that is safe is to place the jam in a jar that’s been boiled in water for a while, leave a little head-space of air, put on the lid and ring, turn the ring somewhat tight, then process by placing the jar into a pot of boiling water, with water at least an inch or two over the top of the jar, for about 15-20 minutes. Time depends on what you’re making and the size of the jar. Then fish out the jars and let sit until completely cooled down. If you don’t hear/feel the lid “ping” down, refrigerate it.
Pasteurization is similar but IIRC at a lower level of heat for longer.
Anyway, follow recipes specifically made for home canning. I’ve bought a few books on the topic, and love the Ball Complete Book of Home Preserving and Canning for a New Generation: Fresh Flavors for the Modern Pantry. The latter is divided by seasons and also includes companion recipes for dishes you can make using your canned goods.