It's hard to compare aches before acclimation to physical work with chronic conditions that develop over years of long, daily sedentary periods. The keyboard being more comfortable on day one isn't the point, right?
Carpal tunnel isn't about being sedentary, is it? It's about repetitive movements (from people I know who suffer from it, vibrations also exacerbate their conditions, which is why I brought up power tools) - just like the common plumbing activities that I mentioned. This isn't about aches. If your body can't handle sitting at a computer, then you are out of your mind thinking that working a trade will somehow 'be better.' Buy a standing desk, ergonomic keyboard + mouse, and go for some walks.
You keep mentioning carpal tunnel but I don’t think that was his main point.
It’s true that white and blue collar work have ergonomic and health hazard mitigations. I saw from another comment of yours that you mostly had to work on unsafe and inconsiderate commercial crews with no decision-making ability, which understandably colors your view of physical labor. You’re also right to point out a novice would likely fare similarly, especially if approaching a career change romantically.