Some claim the same thing for 200mm vs 300mm, that a 70-200 f/4 IS at 200mm, for instance, easily upscales to something better than any 300mm non-L to bring more detail. Not even close. Sure the 100% view looks sharper than from a 70-300 non-L or tamron 70-300 VC or a sigma 70-300 etc. but the actual total captured detail doesn't match the lenses that hit 300mm. Even the old 75-300 IS disaster would bring in a bit more total detail (although the hazy lack of contrast and CA and so on might be enough in that case that you wouldn't care that it brought in any more total detail).
OK, this is very debatable. I was about to wade in and say 'look at this':
But, or course, with the 70-200/4 IS cropped in to 320mm equivalent, its now got 18MP of 60D resolving power behind it instead of just ~8MP from your typical FF when cropped down to that field of view.
However, what this does highlight is that this sample of the 70-200/4 IS can resolve more fine detail when cropped to the same AoV than this sample of the 70-300 non L does natively. All you need is enough pixels behind them both to reveal each lenses potential. And, of course, sample variance might mean that other 70-300's are better.