I am not as convinced making a t-shirt for your startup as a first thing is a great idea. I think that can actually be an indicator of playing house. But you can do whatever you want.
shirts can be printed on demand for less than $35 at MoQs of 1. Its not a large expense and it builds some camaraderie on a small and newly formed team.
People like symbols and having artifacts from a specific time in their life. Get the tshirts/hoodies, but don't spend more than a couple hours picking a design.
I worked on a musical once where the book writer did photo shoots, logos, and T-shirts before the actual script, and it was a sign of bad things to come. So I agree with you and think this advice is mainly only a thing if you're on the level of Cederholm.
As an aside, I like the idea of ending a response with "But you can do whatever you want." I'm going to end every work email today with that!
It is described as "...because while imitating all the outward forms of starting a startup, they have neglected the one thing that is actually essential, which is to make something people want."
