Besides the idea that finance (i.e., ROI) is relevant at all to the decision to read a book, this doesn't even work as an ROI analogy. You're comparing the investment by one person (the person who writes the work) with the "return" that someone else gets (the person who reads it, though calling the fact that they can read it in less time than it took to someone to write it is hardly a "return").

All this has nothing to do ROI, which measures the return that a single entity gets from an investment they make. If you want to make the ROI analogy work (which I don't think you should) one way to do it would perhaps be to compare the time invested with the amount of people the writer reaches. In that case the "ROI" of any method of writing could be super low (if nobody reads it) or super high (if everyone reads it).

Another way to look at it would be that the time to read something you cite is actually the amount of "investment" by a reader. In that case I expect that many readers experience zero return (or even negative return) on their investment of 30s to read an internet post, while they may experience incalculable, life-changing return from 8 hours spent reading a book. Or, probably less often, vice versa.