Those forum answers are evidently ill informed. Assuming the general design of all Macbook chargers are about the same, take a look at this teardown:
http://www.righto.com/2015/11/macbook-charger-teardown-surprising.html
and see note 12 near the end. I tested my own 2008 Macbook Pro charger and superficially agree - I measure 1kohms between AC ground and Magsafe ground (I didn't open my charger to check for the 9.1Mohm resistors) when using the grounded plug cable. My device has USA plugs.
I do not know the engineering reason for this, when they also supply the ungrounded "duckhead" 2 blade plug, which, I assume, also meets all safety regulations. My wild guess is that some countries may require this and not supply a 2 blade adapter. Or if someone in the USA complains about feeling leakage buzz on their device, they may recommend using the grounded cord.