Julie, your assessment is reasonable. The detail perhaps not captured is the ease of set-up with the Domino. Yes, you could cut tenons with the slot mortiser, but it will require set-up, test fits, tweaking, etc. The Domino is really designed for perfect-fit, dust-free, high-strength jointery of essentially any pieces of wood in a matter of literally seconds.
This is one of the best "how to" manuals for the Domino I've ever seen, and there are some incredible examples of very fine furniture being built using very unique applications of Dominos. Scroll through the pictures, at least, to get an idea for how it is much more than a doweling system.