No, the Python bug is genuine. When there are flags which have no value, but are just ‘defined’ with name, say -D DEEP_SLEEP, then those values are not decodable as key,value pair.
This is however poorly written in the documentation, in which the example
my_flags = env.ParseFlags(env['BUILD_FLAGS'])
defines = {k: v for (k, v) in my_flags.get("CPPDEFINES")}
is displayed. In this example it really works because all flags happen to be of key-value form.
I’ve also noticed that a year ago in Change name of firmware file - #10 by maxgerhardt where I wrote a better version which first checks whether the instance is a list (aka key,value) or just a singular value / name. An issue in GitHub - platformio/platformio-docs: PlatformIO Documentation should be filed to improve the documentation in these cases