Yes, sorry that is correct.
Their site... well, it’s certainly not how I would have done it, and I can’t actually comment on the positive or negatives of their actual code... as of yet, I have a client that will hopefully eventually (you know how those go) be developing a site that I was intending on using their script on.... however if MODx comes out with one before then, perhaps all the better!
As an experiment, I did drop their script into an existing site layout, just to see how quick, down-n-dirty it could be, and it was pretty slick.... literally copy and paste a few lines at the top of the page, about 5 lines of code into the main content area, then change a couple of those lines based on the individual page the code was going into, and that was it.
Obviously, fine-tune styling of the cart would be necessary, or recommended, but you could be up and running at that point if you wanted to be, after setting up the configuration within the manager. But there are things that I think are really missing, like adding product photos is done by entering in the path and filename, and the images don’t appear within the admin product setup page... although there are addon modules that can accomodate both of those features as well as adding the ability to add more images.
But I mention that solution more as something to be looked at for development ideas for a MODx cart, than actually integrating it into MODx. From what little look around in their code, I think html presentation and php are intertwined a little too much.... but given the genre, show me a cart that isn’t

This is of course where I think MODx could improve upon greatly, and quite possibly, indirectly, end up providing not only one of the best CMS/F solutions, but also one of the best ecommerce carts as well.
I’m no cart expert and there are plenty here who have far more experience, I just know that in my limited searching and reviewing, the bulk of what I found was "add this alongside your site," "change your site to match our cart," or "try to stretch your site over the top of our cart" type solutions and ECT was one of the few that, like MODx, said, "here are some designs you can use, but you are free to easily create any design you want, or drop our functionality into your existing design."
I don’t know what really separates them from all the others, but they must have approached things differently that you can so easily drop their code into existing designs where all the others seem to be a never ending battle.
Certainly from what I’ve heard and my own looking briefly at Zen Cart and Cube Cart, they come well recommended... but as mentioned, they are trying to be the Swiss Army knife of ecommerce carts (like nearly ever other cart out there) and it ends up being the issue of taggling off this feature and that feature, but it’s all still there, a ten pound pocket knife you end up carrying around, rather than starting with key functionality and adding on the additional components or functions that you need... which I see much more aligned with MODx. I didn’t ever install Zen or Cube, so have no idea how easy they are to integrate into a design, but the impression I get is that they may be easier than others... but ease is still a very relative term.
Since ECT is a paid solution, as are the add-on modules, it certainly wouldn’t be an option for everyone anyway.