I assumed this was so they could serve a "lite" version that took the opportunity to cut down their outrageous JS overheads for the slower connections and devices. But digging into the code there's no evidence that this is so. It's just a slightly different template with adjusted inline styles.
I suspect this approach is a legacy from a time when it wasn't so clear how best to handle mobile users. Or, perhaps from the limitations of the editor, which may have been written years before the dominance of mobile.
Either way, it creates a number of issues that complicates the editor.
For instance, the desktop version has practically zero responsiveness too. Content doesn't flow normally or resize dynamically, instead almost everything is positioned absolutely.
This means adding content or swapping two content blocks can be a delicate act of manual drag-and-drop repositioning, that litter the HTML with inline styles like this:
margin-left: calc((100% - 980px) / 2);
This 980px measurement isn't arbitrary. It's a user-defined value that marks a safe area. Some content blocks positioned or sized beyond this boundary (marked by snap points in the editor) will be cropped if the screen becomes narrower than this width. But then, others won't. It's a mystery that I didn't have the time or patience to uncover.
Another result of all this absolute positioning is that the page footer can't be just another element in the document flow. Its position has to be dynamically calculated, with styles like this:
position: absolute; top: 15240px; height: 215px;
They don't even bother to put the footer at the end of the DOM. Why bother, when it has to be manually positioned? Sadly, this breaks keyboard navigation. Tab through the page and focus will jump to items in the footer before items in the body.
Maybe this is an issue with the specific template I chose, but it seemed to me that the editor is geared around this kind of brittle design. It's a nightmare.
All of the above problems are compounded with the entirely separate mobile design. You'd imagine that, considering single-column layouts are so much easier to make responsive, surely this stand-alone template would stretch out to feel spacier on larger screens.