Mind you, the GPC lists the meaning of “ar gyfer” as meaning “(i) in front of, over against, opposite to, facing; corresponding to (of time, day, &c.); for, in preparation or in provision for; in respect of, with regard or reference to.” which covers both meanings as dating from the 12th century.
I have no idea, without access to the print version and the examples therein how this plays out.
Unfortunately, Welsh dictionaries (including even such things as the Geiriadur Mawr) were sometimes- erm, comparatively speaking with large languages (though doing well for a small language), always- rather wanting until the GPC and Geiriadur yr Academi in the twentieth century.
Pains me to say it, but it is true. Welsh dictionaries were written by well educated and enthusiastic individuals. Which is not enough for a decent dictionary covering the entire language.
[and even in the twentieth century, the ease of spread of information was important as there was still a minimum number of people… Oh, never mind ]
This does not mean “ar gyfer” has not changed its meaning over the past two hundred years. It may well have, and no reason why not.
But unfortunately more than a pinch of salt is required when using earlier dictionaries. Not because the people were uneducated or bad scholars- quite the opposite, considering what they were working against- but because dictionaries, considering the vast field they cover, are one of the few things on this earth that are best written by committees- and until recently, and still to more than a little extent now, “British” universities would pay for committees for such things in the English language, but not in the Welsh.
Unfortunate, but true.
[oh, and edit- it’s not just the “wales within Britain” thing that this works with- all smaller languages are affected by this. All languages are as wide encompassing and vast as each other, but smaller languages have, obviously and unavoidably, a smaller number of people working on as large a coal face as larger languages.]