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.
"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.
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.
less than (seven) (millionths of a second)
less than (seven millionths of a second)