You've dug pretty deep into the recesses of InDesign here. What "Resize Horizontally" refers to is the Object based Liquid Layout functionality when using the Liquid Layout feature in InDesign. If you select the Page tool and open the Liquid Layout panel (Window > Interactive > Liquid Layout), you'll see that the shortcut for the option you are inquiring about turns on the Width property for "Resize with Page". It's a pretty obscure feature but basically controls how an object resizes when the page is resized using the Page tool.
Opened Liquid Layout panel (selected Object-based layout) and selected the Page tool, even tried going from facing pages to single pages, still don’t see resize horizontally, or vertically, highlighted (screenshot below)?
At this point it’s merely for curiosity’s sake, these features fall outside my current workflow.
Thanks for providing clarity at any rate. Cheers.
Liquid Layout Panel open (Object-based selected), Page tool selected.
Got it. Yeah I kept merely focusing on the page tool itself without trying any other tools or the move tools.
This lead finally helped me to witness the key command in action.
We are always happy to help.