I would say you described "one seven millionth" of a second (1/7,000,000 s)
"Seven millionths" would be 7/1,000,000 s (7μs). They take 20 to 40 images in that period using 7 cameras, so any given camera might be as low as 1.4μs per frame.
Yes, but they said seven-millionths of a second, not seven millionths of a second. Technically they're right that that's what it means, but I'd expect an editor to recommend against that phrasing in favor of the one you used to avoid confusion.
Well, it's true that the article says "seven-millionths".
I would guess it's a lot more likely that this is an editing failure, introducing a hyphen where no hyphen should be, than that they meant to divide a second into seven million equal parts.
For one thing, as SECProto alludes to, English would normally require you to say "less than a seven-millionth of a second" if that was what you meant. There's no such thing as saying "less than weeks". You have to specify less than how many weeks.
"Seven millionths" would be 7/1,000,000 s (7μs). They take 20 to 40 images in that period using 7 cameras, so any given camera might be as low as 1.4μs per frame.