[carpdad]

It says 16d face nail in studs. Probably same into the foot plates and rest is by gravity. IMO, I would never use lumber below grade, treated or rated, unless the area was desert conditions. It shows gravel and sump pump but gravel can fill with silt over time. Water pressures make it IMPOSSIBLE to contain it. This forum has few posts where wood foundation failed. Internet also many examples. Even 2x12 is only nominal 12" and lumber can crack and split. Lumber studs and sheathing can fail. Nails pull out from lumber. Screws unscrew.
IMO, that is not a diagram for a full size garage but maybe a small shed. Earth also moves and gravel pit can move. Slab may sink. Why bother? Internet is full of interesting/innovative ideas but that is not intuitive idea even for me.

 

[carpdad]

NY so you need deep footing. Not a mason so my thinking is shallow. Why not dig 24" trench, don't use forms, if must just plywood facing against dirt wtih braces and bury it with footing. Then short forms for relatively short wall should not be big problem. Even if you're not a mason and diy this foundation, I don't think it is impossible to do in a narrow space.
There is youtube video who built shallow footing slab house. He's in NY. Shallow footing is becoming accepted though I would not, but I don't really understand it.