They do more than that. If you ask for ui with a button that button won't be upside down even if you didn't specify its orientation. Lots of the detail can be inferred from general human preferences, which are present in the LLMs' training data. This extends way beyond CS stuff like details of algorithm implementations.
That’s exactly what they said. Details “elsewhere in its training set”.
Isn't "not being upsidedown" just one of the default properties of a button in whatever GUI toolkit you are using? I'd be worried if an LLM _did_ start setting all the possible button properties.