It doesn't sound like Mann is disagreeing with the core mechanics of GTD in those posts, it sounds like he is saying that there are diminishing returns with applying 'personal productivity' methodologies, and you can get obsessed with tweaking your personal workflow to an extent where you no longer get payback.

This is a trap I have fallen into before - where you are effectively procrastinating by trying to optimise and system-tweaking your personal GTD system rather than just actually-getting-stuff-done.

Having said all that, GTD helps me personally massively. It's not for everyone, but for lots of people in senior jobs that gets so much 'stuff' in that they struggle to keep up, it's really important to have a systematic way to process it all and not drop anything. I find people who don't have a system like GTD or similar will tend to work from their inbox and end up dropping stuff.