You must separate the game from its presentation when considering game as art. Chess figurines can be artful, but that does not mean that the act of playing them becomes art. That same separation translates to Braid and The Witness: how is requiring to press a button to reverse the player actions in a specific way in itself art? I'm deliberately separating that from the notion of "time reversal" here because that's how that action is embedded into the game's presentation. The actual physical flow of time for the player in the real world is unaffected by the button press; so this separation is justified. Therefore, the final flow reversal from the phantasy to the real plot in the last level is just a twist that happens in the presentation and not a consequence of the game rules.
The Witness is a slightly more interesting case in that it presents a sequence of evolving puzzle rules. The rule evolution again relies on presentation aspects to achieve the "twist": early puzzles are clearly framed to stress the notion that puzzles are limited to clearly discernable canvases. Later puzzles dissolve these boundaries until they are no longer present and the puzzles are simply part of the environment. This sequence is only partially gated by the game (whether or sequencing/gating in a game is part of the rules or the presentation is another can of worms). The only real change from clearly framed to unframed puzzles here is that the problem statement transforms into finding the right viewpoint. Which part of this is art?
How do you define art, then? What is the art in a Kandinsky painting?
In my opinion, the art in these games is the beautiful intersection of a set of puzzle rules with the world they inhabit. In The Witness, the world is additionally constructed to be thematically interesting, and I think that the set pieces on the island are intrinsically connected to the puzzles next to them. The island by itself would not make for very interesting art: it’s the game element that brings the art to life.
Also, “pressing a button to reverse time” feels very reductive. By playing the game, the player learns, and this learning leads to understanding. Usually, this understanding is tied solely to the game world and is used to tackle new and more complex challenges in later areas, but maybe it can also lead to extra-game insight. Even a tiny game like Passage makes powerful gestures in this direction.
My position is mostly based on Brian Moriarty's Apology for Roger Ebert, by the way. I also define art mostly the presence of an intent that an artist wants to convey to the audience that goes beyond the immediate form.
Somewhere along the path towards modern computer games, a conflation happened between the game and its presentation. My intention here is to point this out. If you retain the meaning of the word "game" from its pre-computer origin, you arrive at the reduction that I outlined and the question whether you can convey artistic intent solely in a set of game rules.
Artistic intent is always somehow relying on a certain level of control over an artwork's presentation. What is (or isn't) shown in painting, how is it represented and where is it on the canvas relative to everything else? Music, film and video have a temporal aspect under the control of the artist. In that sense, Kandinsky paintings can be art under the assumption that the compositions of these paintings are very deliberate.
In a computer game, there is always tension between the game elements which give agency to the recipient and the presentational aspects for which the creator has to assert control from the player in some form to ensure that they are conveyed with the proper intent.
This tension makes any answer to whether video games can be art nontrivial.
The Witness is a slightly more interesting case in that it presents a sequence of evolving puzzle rules. The rule evolution again relies on presentation aspects to achieve the "twist": early puzzles are clearly framed to stress the notion that puzzles are limited to clearly discernable canvases. Later puzzles dissolve these boundaries until they are no longer present and the puzzles are simply part of the environment. This sequence is only partially gated by the game (whether or sequencing/gating in a game is part of the rules or the presentation is another can of worms). The only real change from clearly framed to unframed puzzles here is that the problem statement transforms into finding the right viewpoint. Which part of this is art?