I'd take that website with a grain of salt, other websites show different results for a 10 digit numeric passcode. The FBI paid a million dollars for a company to crack a 6 digit numeric passcode on an iPhone 5C and it took a lot longer than 3 seconds.. You have to defeat the 10 attempt passcode or erase all contents first to use brute force. Apple is constantly patching vulnerable ways to attack the phone so I guess if the government wants into your phone it's different than some random guy that stole your device... 10 digit numeric passcode is way more secure than what 98% of iPhone owners use.
So the difference must be the hardware. Because I've always, always read that security experts recommend long, randomized passwords containing all categories of characters (upper, lower, special, and number) when it comes to websites and such. So I'm guessing a 6-digit code on an iPhone is far more secure than a 6-digit password on, say, PayPal. Somehow I doubt it would take a million dollars to crack a 6-digit password on a website.