I interpret their comment slightly differently; Factorio as a game is less valuable today then, say, 4 years ago.

I still disagree with that interpretation, as the game has continued to receive updates and bugfixes, steadily increasing it's value (or at least counteracting the depreciation). Not to mention the additional value provided by community mods has only increased over the years.

The game is also one-of-a-kind. Until a "factorio 2" equivalent comes out that is just straight-up better in every way, it's hard to see how the value would depreciate. Heck, the Space Age DLC is basically "Factorio 2" without splitting the playerbase across 2 separate games.