I don't agree. He could simply say "After reflection and thinking about what I want to do next in my career, I've realized it's time to part ways with Canonica

I don't agree. He could simply say "After reflection and thinking about what I want to do next in my career, I've realized it's time to part ways with Canonical.". Frankly, that'd be about as detailed as what he posted, but without the parting shot (it's not like he elaborated on his management complaints). He owes no one a detailed explanation for his decision and it's perfectly normal to keep things like this close to the vest.