Hi, thanks for your work. Regarding: "I’ve listened to this bit many times and I’m pretty sure Tesler says ‘foils’, and I guess he’s using the term in its architecture‐related meaning of “a leaf‐shaped curve formed by the cusping of an arch or circle” — But I welcome any correction from native English speakers."
He surely talks about preparing the "transparency" foils for presenting them using the overhead projectors:
https://en.wikipedia.org/wiki/Transparency_(projection)
"A transparency, also known variously as a viewfoil, foil, or viewgraph"
That's how the presentations were done at that time, using the:
https://en.wikipedia.org/wiki/Overhead_projector
The good side was that you were able to prepare the foils before but then annotate any (i.e. write or draw on it) during the presentation, which had its own dynamic -- it could have been a result of the interaction with your viewers -- like while you were answering some question.
The foils were even used in the very talk you transcribed: https://youtu.be/OW-atKrg0T4?t=1295