That's a labor theory of value and, thus, inaccurate. PoW merely ensures that one's (monetary) interests are best served through a coin and not against it (for the non-trivial investment that is the common PoW-coin mining operation).
Time + space = non-trivial investment
What's being gotten at, however, is that the investment itself does nothing to bolster the assorted (sensible) valuations of a PoW-coin: it prevents the coin from coming under attack by malevolent-thereto miners (or, in other words, "crypto- assassins").
What you're missing is that if you are evaluating the non-trivial investment, you can't disengage it from what makes it a non-trivial investment if you want to do a an honest (thorough) evaluation.