Not a structural, but curious about this.
Why would anyone suggest that an addition to an existing slab on grade garage be made with anything other than a slab on grade foundation? Not saying it's wrong, just wondering what the reasoning would be, and how it would work? Doesn't a slab on grade move with the frost heave, at least a little? And if so, in either case, how would the tendency for the two slabs (original and addition) be encouraged to move together rather than independently, distorting the building they're supporting?
Based on some research I did for a neighbour, I do know that thickened perimeter slabs on grade with suitable reinforcement are acceptable for detatched garages in my locale, which has significant frost depth. Built without a stem wall, this is very much more economical to construct than a perimeter wall and slab construction would be.
Whether or not you will get any benefit out of a stem wall around the perimeter on top of that slab on grade depends on the grading around the structure, whether or not you want any slope on your garage slab, and what you want the finished building to look like. If the grading is good, you don't need it, though you may want to specify a pressure treated sill plate. If the grading is poor, your framing won't survive long term without it. But I can say from experience that framers hate having to stand stud walls up on top of stem walls, especially when the anchors are already set into the concrete- it's a royal pain. Framers here far prefer to platform frame a structure rather than to stick build it in place on top of a stem wall, toenailing all the studs to the bottom plate and then sheathing it when it's standing, because platform framing (framing, squaring and sheathing a wall on the floor and then standing it up) is way faster and safer and easier.