> Besides, during the 19th century average height increased enormously (a sign of good and consistent nutrition), mortality rates dropped dramatically, life expectancy increased, and the growth of the middle class was spectacular.
Growth is good, but it's a delta: it doesn't mean that the average life was particularly nice by modern standards. The fact that things improved over time is not evidence that they were good at any instant (and indeed, nobody questions that things improved: they clearly had to in order to get to where we are now).
In 1914, the average male life expectancy was 52[1]. That's over 20 years shorter than our current life expectancies.
> When the US entered WW1, the German soldiers were shocked by how tall they were, and how well fed and well supplied they were.
This is a comparative error: the US's military population didn't need to be well fed by objective standards; they only needed to be better fed than the peasants who were press-ganged into the German army.
Nobody is saying that the US was uniquely impoverished by contemporaneous metrics; the claim is that the US was impoverished on an absolute and modern scale, and that said impoverishment declined as the US established domestic and international policies that are inextricably linked to its monetary policy.
[1]: https://u.demog.berkeley.edu/~andrew/1918/figure2.html