...I think he mis-typed when stating it's at 300mm - I think it's at 70mm and the rear element moves further 'in' (away from the lens mount) as you zoom to 300mm - at least, that's how most extending zooms behave...
Well, yes and no.
a) I'm bored.
b) It just started raining so i've got nothing better to do than take photos inside.
c) I just got my first-ever speedlite in the post from ebay (a 430EXmk1) that I wanted to try out, i put it on the floor pointing at the rear element to reflect to make it easier to see.
d) The 70-300L is not a "normal" design, it's got 'floating elements' all over the place. At 70mm, the difference between macro and infinity is not much. Leave the focus at macro and zoom to 300mm, the rear element will come out a bit (200mm or so) then go back in by 300mm. At 200 - 300mm, the difference between macro and infinity is more than a few cm travel into the body.
e) In a way, that article is right. you *can* physically mount the 2x TC onto the lens, there's nothing stopping you. *BUT*
you can only focus at infinity, and above 200mm or so. Focussing any closer than 5m @ 300mm, or infinity @ <200mm (a guess because I don't have one in from of me) will be crunching stuff. He may have meant "it was obvious I would have to shoot using manual focus" because of the aperture, but also because if AF hunts it's going to break something.
f) It's probably ok to use a t/c if you put an extension tube in between the t/c and lens. But then you're at macro only.