This isn't testing what I thought it was testing. I'd forgotten that
format(x, '') is exactly equivalent to str(x).

The better test is using a format string of '<':
>>> 1e200.__format__('<')
'1.0e+200'
>>> 1e200.__format__('<g')
'1e+200'

So this is a non-issue: the current behavior matches the PEP
description. The online docs could still be tweaked, though.