This commandment is one that I’ve basically adopted in whole cloth from Sid Meier (the game designer behind Pirates, Railroad Tycoon and Civilization among others)! To see some more thoughts on the originator of the idea, check out this sweet design article by Soren Johnson.
This idea is fundamentally tied to a management of complexity/elegance of design. Many games (such as Sid Meier’s!) take their cues as at least partially a simulation of real-world systems. This has the twofold advantage of giving the player a reason to play your game (if they’re already interested in the subject matter) and some basic idea of the way things will work, without you having to explain them (related: Mark Rosewater’s thoughts on Piggybacking and Keith Burgun’s thoughts on theme).
However, strategy game designers need to be include to select only those elements which make for interesting player-facing decision-making. For example, in a war simulation, it may be tempting to include mechanics that revolve around supply lines, solider morale, etc etc. And then you may think that hey (!) I can tie the solider morale into the combat by giving certain bonuses, and cross compare that with the unique background of each solider, and so on and so forth.
How does this even work? What should I care about?
While this may provide value in the exploratory framework of a simulation game like Dwarf Fortress, where the value largely comes from discovering what the mechanics are and what emergent results they produce, it does not provide efficient value in terms of strategy game decision making. It may be very interesting for the designer as they set up the various status tables, modifiers and relationships between them, but so long as the player is not aware of or not considering them as an aspect of their decision, they are not a part of strategy (and therefore the fun/value of playing a strategy game, for its strategic merits).
By including a smaller and more carefully curated set of mechanics in your strategy game, you allow the player to get done with learning the rules as quickly as possible (as fun as they are to design and tweak) and move on to the joy of strategic play! (See also article 1 about Known Rules)