I don't buy it. Python carries more goodies, more cargo. The clipper goes the same speed in supercruise and has less internals for everytning, fewer hardpoints and a bigger ship.
Maybe in a single run a clipper can get to 100 minerals first but on any sustained mining the python makes fewer trips to unload and can have nice things like a subsurface missile or something.
Thing is, if you only consider the things that make the Clipper "the best" at DC mining, you
could go on to argue that an iCourier or Viper is even better.
I doubt many people will be using those ships, though, because there is obviously a broader, and more subjective, list of requirements depending on what each player is doing.
To me, it's kind of like trying to argue that a Lamborghini Huracan is "the best" car for going on holiday because it'll get you to your destination the fastest.
That might be objectively, undeniably, correct IF you're travelling alone, on good roads, with a small amount of luggage.
If you've got a family, or you're taking a heap of camping gear, or you're driving on bad roads, a completely different car will be better.
If somebody wants to continue to insist that the Huracan
is "the best" car for going on holiday, based solely on their own criteria, it's probably not worth bothering to try and convince them otherwise.