You stated a bit ago:
I do think there is an interesting discussion to be had about what constitutes "play" in a broad sense, but my overall thesis was directed at "play" meaning actually sitting at the table doing the thing. I probably should have more rigorously defined that.
So substituting that definition in the thread title leads to what I assume is your position:
"Actually sitting at the table doing the thing is paramount"
You alos state that you want to narrow the definition of play and mentioned char gen as something that was
not "play" in your definition. I'm 100% on board with this, otherwise pretty much anything can be counted as "play" or enhancing play: Playing football helps me understand how bruises feel so I can better describe combat, which enhances play. Without the narrowing you are looking for it is hard to think of anything that enriches a person's experience that does not enhance play (barring things like losing a hand ...)
Again substituting your definition of play into your statement about play, you state:
The thesis does not imply not doing that kind of preparation; it demands that kind of preparation is aimed at actually sitting at the table doing the thing, not done just for its own sake
My contention is that a lot of value can be found in tangential activity -- activity which helps "play" in the general sense, but not "aimed at actually sitting at the table doing the thing". Tolkien did not invent the linguistics of Middle-Earth
aimed at writing LOTR, but his book is so much the better for that work having been done.
As an anecdote, I watched every single related piece of media I could think of before running my Dracula Dossier campaign. The goal was not aimed at the experience of sitting at the table playing the game, but to condition my thinking and ideas generally to make the game better. I absolutely could not defend my watching a Canadian ballet version of Dracula as aimed at running a tables session.
A secondary contention is that if you are running something short, you can -- and probably should -- aim at actually sitting at the table doing the thing. Prep that session events, learn the rules, build some props. but if you are running a longform event, like DD was for me, it is important to be tangential and build up a set of knowledge and capability that will enhance the play at the table, but is not
aimed at that objective.
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TLDR: Virtually everything helps play in the general sense. If we narrow to activities aimed at the et-table experience, I think there are additional activities that enhance the at-table experience, but are not aimed at that experience, and that in long campaigns, those "tangential activities" are of high importance. Possibly paramount!