Thanks for the kind words whacked_new!
I agree entirely with your prediction: spreadsheet programming is different from interactive "control flow" programming... and it should remain different! The spreadsheet model of programming is easier to grasp and incredibly flexible. I think the key is that spreadsheets are time-less & space-full, I hint at this in the talk & see also my root comment for more elaboration on this.
What's exciting to me about lambdas is that they can be used to enhance this time-less, space-based style, like in the examples I elaborate around 18m47s: the habit tracker, timesheets & a spatialized Game of Life.
I imagine/hope that in a few years we will have a culture of casually importing magical custom functions that will become polished through mass use and eventually become standard. These native functions are much safer than macros so you can be as casual with them as copy-pasting formulas. There's no permissions fuss to wade through, as there is for AppScript. (I even think there will soon be libraries of custom functions, I might even publish one myself in a couple of months.)
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As to your question, I interpret it as: why are lambdas coming inside spreadsheets instead of spreadsheets becoming incorporated into programming languages, right?
I have come across many extensions of spreadsheets over the years. Spreadsheets are so ubiquitous there's many kinds of wrappers for them, many hybrids. Since the birth of spreadsheets in 1979 there has been a flood of drastic alternatives & variations that have been tried. The most famous variant so far has been pivot tables, which has become a useful, somewhat infamously intimidating feature of spreadsheets but not the revolution once expected. One of the more interesting extensions is TreeSheets https://strlen.com/treesheets/ But nothing yet has beaten the simplicity & power of a flexible grid sprinkled with formulas & references through the value rule.
To me the most important thing I learned researching this talk was that it crystallized in my mind that spreadsheets are not just a table, not just a grid. They have a key constraint, they intertwine code and data through the value rule that Kay pointed out 5 years after dynamic spreadsheets were invented (see http://worrydream.com/refs/Kay%20-%20Computer%20Software%20-... ).
So my answer to your question is that spreadsheets have been incorporated into programming languages, they have long grown scripting languages like VBA or Google AppScript. Those are massively useful for top-down extensions of the functionality of spreadsheets.
But the other direction, lambdas in spreadsheets, has been much slower & difficult, yet it offers the promise of a more bottom-up revolution in usage. It has taken us decades to evolve the model of spreadsheets towards lambdas in a way that works well with their true nature and to the ton of other features & culture they've grown over the years. (My personal favorite feature, btw, is the incredibly flexible formatting we can give to a sheet, specially conditional formatting.) As mentioned in the talk, spilled arrays are a recent, crucial step to make lambdas possible & useful.