You are clearly entitled and more than qualified to believe in and create old wife’s tailsYou gotta remember that was originally written back in the Dark Ages and I was 1. Using information other baker's had posted, as well as 2. Personal experience. Maybe the metal wasn't killing the yeast but it most certainly was affecting the way the yeast was working -- and not in a good way. Finally 3. I dunno anything about what kind of metals they used to make bowls and spoons back then as opposed to what they use now. Same for any kind of coatings.
Also ... I know plenty of professional bakers who use metal utensils and equipment so again ... AND ... There are different strains of yeast. Sourdough is especially finicky. I've NEVER been able to get anything with a sourdough starter to do anything other than croak like an old hag when it's come anywhere near metal anything -- spoon, bowl, baking pan, you name it. Maybe somebody else has had a different experience but I'll stick with my glass and wooden stuff when I'm playing around with sourdough.