Yeah this part is a bit ridiculous. A good reason to avoid "str" is because something more descriptive (and probably not much more verbose) would be helpful to see instead. "Because it's overused" is a silly reason, I would hope the author merely intends to criticize inappropriate overuse of these minimally-descriptive placeholder names.

When I'm mapping an array of line items with lodash, the predicate parameter is usually named "item" because it is short and perfectly adequate in context.