I've re-thunk this since first posting ;)

Best guess is he just used natural light, not through his kitchen window, as I initially had assumed, because he states he didn't take it to the kitchen.

However, in the 1920s I would assume an artist would have an artist's loft*, with high, broad natural light... & a photographer would use one too, for similar reasons; that, combined with the reflected light inside the tin funnel the pepper was placed in & a 6-minute exposure, appear to have given him all he needed.

From Wikipedia - Pepper No 30

He first tried with plain muslin or a piece of white cardboard as the backdrop, but for these images he thought the contrast between the backdrop and the pepper was too stark. On August 3 he found a large tin funnel, and, placing it on its side, he set a pepper just inside the large open end. He wrote:

"It was a bright idea, a perfect relief for the pepper and adding reflecting light to important contours. I still had the pepper which caused me a week's work, I had decided I could go no further with it, yet something kept me from taking it to the kitchen, the end of all good peppers. I placed it in the funnel, focused with the Zeiss, and knowing just the viewpoint, recognizing a perfect light, made an exposure of six minutes, with but a few moments' preliminary work, the real preliminary was on in hours passed. I have a great negative, ‒ by far the best!"

By placing the pepper in the opening of the funnel, Weston was able to light it in a way that portrays the pepper in three dimensions, rather than as a flat image. It is this light that gives the image much of its extraordinary quality.

One thing about still life, which I do a lot of myself, is it never gets bored.
It leaves you to do all the thinking. It doesn't shuffle its feet or want a cup of tea, & blur your image if it has to hang a round a while. A flower stalk wilting slightly over a focus stack taking 5 minutes is about the worst you'll have to deal with.

*There is an entire row of these houses near where I used to work in London, all designed North-facing but with huge windows to admit lots of broad, indirect light. It's known colloquially as Artist's Row, in Chiswick, West London..

Pic, Credit:© Philip Ide Photography

I also found a newspaper article in the Daily Mail with pictures of the interior of one of them - can I have one please??