This is one detail the author got wrong. What she called “bonnets” were actually prayer coverings and in most groups, that means a thin white or black fairly sheer fabric. Their actual bonnets, which resemble the old sunbonnets, are worn when they leave the house to go to town, or church.
This varies so widely among the different Amish groups that one simply cannot make a blanket statement. In Ohio, some church districts require girls who have not yet joined the church to wear black coverings, but some don’t.
https://amishamerica.com/amish-womens-head-coverings/
Thanks for the info. I didn’t think that statement was correct. About certain issues, the Amish have always seemed rustic but quite practical.