I think that may be the most roundabout and head-spinny justification for self-deception I've ever heard.

Note that I don't endorse self-deception, see my other comment in this thread. But the argument points to a negative trait of the choice. (The argument is related to a stance that as a rationalist, you'd want to use rhetoric as much as is common (but not more), to avoid signaling the incorrect fact of weakness of your position.)

By a similar token, should I not take up gardening if it's not within my power to update everyone who has the belief that I don't garden?

Normally, if you take up gardening, other people's level of belief will either be unchanged (prior state of knowledge: they don't have new evidence), or will move up (towards the truth) upon receiving new evidence. Here, the situation is reversed: new evidence (not new action -- this is a point where your analogy breaks) will move people's belief away from the truth.