[thaumasiotes]

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.

    less than (seven) (millionths of a second)

ordinary grammar, ordinary unit choice

    less than (seven millionths of a second)

improper grammar, bizarre unit choice.

[thfuran]

I agree based on the whole sentence in the article that that was probably an editing error.