This is just a guess without seeing the full tutorial or uncropped interface, but I'm pretty sure they just set their Document size to a square size (eg: 512x512). The size or ratio isn't necessary, it just helps if you plan on exporting a seamless, square image.
What they're going is drawing the branches onto the Document like normal, but they're NOT using Edit mode (except to tweak their size and positions of the Tool onto the document). Otherwise they keep Edit Mode off, which turns the branches into regular image pixols.
When not in Edit mode, you can scroll / offset the current Document Layer like that by holding ~ and LMB while dragging your mouse.
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Thanks for your help!!