Modding basically recoups the investment into making a game's community - an individual can present their result to an existing player community directly, with a brief forum post, and a small website with some images or video to advertise. That is enough to build a small empire on, going by the most popular examples.
That's different from the typical lifecycle of a commercial game, which has a deeper level of marketing work to do before it can even consolidate a playerbase. There isn't a "Unity modding community", per se. There is a community of Unity developers, and players of individual games made in Unity, who have no broader associations. A game transitioning from the mod sphere of an existing game to an independently produced commercial title still has to overcome this gap, and most of them don't make it over the line.
The advice is sensible, nevertheless. If you can extend the schedule to focus on the core elements of the design as a "R&D" process where the majority must be thrown away, and only scale it up towards a shipping product as the concept proves itself, you minimize the risk of the budget being wasted on a flawed concept. Modding scenes, game jams, and micro-budget productions all have the benefit of weeding out most of the really early, risky design experiments, without wasting enough of people's time or money to care.
Even when conceiving the marketing for a larger production, the same advice works. The "trial balloon" or "landing page" method, etc. You still do design thinking when you market, but it's design on the topics of "how do we build a funnel" or "how do we make this a franchise." Still very easy to spend a lot of money on making a splash without getting the blueprint right.