tl;dr– Since there's no meaningful distinction between an omnipotent, omnibenevolent God who can do evil vs. one who can't, either framing can be applied without contradiction.
This is a question of frame selection.
By analogy, is the glass half empty or half full? Is a fully transparent object more red or blue? If your glass is fully empty, then is it an empty glass of water or an empty glass of tea?
Let's consider some scenarios in which something behaves predictably:
If a rock is dropped, it'll tend to fall. Does the rock fall because:
If a green plant sees more sunlight in one direction, the plant will tend to orient itself in that direction. Does the plant reorient itself because:
If food is placed near a hungry animal, the animal'll tend to eat. Does the animal eat because:
If a person is offered something that they want under terms that they find perfectly agreeable, the person'll tend to accept. Does the person accept because:
If an omnipotent, omnibenevolent God acts, they'll tend to act in a good way. Does the God act benevolently because:
Note: Don't want to make this list excessively long, so it has fewer entries than I'd otherwise put in it. My point was to slowly walk from controlled-by-determinism to controlled-by-free-will to demonstrate that there's no sharp dividing line between them. The final entry is as from the question statement, to tie the topic together.
Scientifically speaking, ordinary rocks could be god-like beings of great power who simply choose to act as they do. No scientific experiment has ever suggested otherwise; then again, science seems to suggest that we can also view rocks as inanimate objects controlled by Physics, and that explanation's a whole lot simpler. So why not go with the simple explanation if it seems just as valid?
The fact that we can see it either way is called compatibilism, as the two perspectives are compatible with each other.
Generally, we choose to see simpler things (like rocks) as inanimate while we choose to see more complex things (like humans) as having intelligence. There're grey areas; if you consider a spectrum of things from inanimate objects (like rocks) to arguably-alive things (like viruses) to simple organisms (like bacteria) to more complex organisms (like insects and plants) up through humans, presumably you'll find a point at which you, personally, would find the distinction between "intelligent agent" and "inanimate object" to be blurry.
Still, since both perspectives are correct, it's not a matter of which is right so much as which perspective is more practical.
Anyway, as for the omnipotent, omnibenevolent God, it's literally meaningless to debate if they
are powerless to do evil; or
could do evil but choose not to;
as these two framings are largely equivalent.
Note: Connotations can vary.
Well, the two framings are largely equivalent to a first-order approximation, so far as the question statement goes in a vacuum.
That said, when folks think of an omnipotent, omnibenevolent God, they typically imagine a God who is intentionally good rather than, say, a God who is frustrated with their own inability to do evil.
So while either framing is valid in a vacuum, it'd be more informative to describe typical notions of God as being able to do evil but choosing not to, as this framing of God as an intelligent agent with a powerful will is what we typically think of when thinking of such a God.
Note: Pantheism frames physics as God.
Pantheism is basically the choice to frame the universe itself as God:
Pantheism is the belief that reality is identical with divinity, or that all-things compose an all-encompassing, transcendent god.
—"Pantheism", Wikipedia [references omitted]
This is little different from an atheist physicist's point-of-view.
Just, as with a glass being half empty or half full, it's a choice of frame rather than a concrete distinction.