Every time the devs insist on using the term "play", a baby dies.
It's got all the definition of an obese elderly woman.
I don't think any rules lawyering by reference to the word "play" is persuasive.
The other terms that they could easily have used in place of it are clearly defined in other places.
Deploy (verb) -> put into one of the three deployed zones from hand, the void, the deck, or command.
Deployment zone -> attack, defend, support zones
Battlefield -> attack, defend zones
Cards that reference "play" or "put into play" has been shown to be riddled with conflicting exceptions, mostly due to exiles or sleepers mechanics. You should just deal with the function of the card as it is, rather than try to rules lawyer the outdated wording.
I don't entirely agree here.
Playing a Character, Location, Artifact or Mission is meaningless, true. But you don't deploy Abilities. According to Aether Acolyte and Siphoner, you actually play abilities (though I'll admit that Altar Acolyte disagrees and I don't have Ancient Aether to check)
In addition to that, "in play" does have a definition. It means the Command Zone, the Deployed Zone and nothing else.
I agree that the word "play" is both overused and misused, but it has a place, however small.