This is the kind of line that I think comes from a throwaway thought, filtered through a game of telephone. Someone says something like, "Belichick and his staff is good enough that they COULD win a Super Bowl with any of the top 15 QB's. Maybe not multiple, and certainly not FIVE, but he's still a SB winning coach so long as he doesn't have a dumpster fire at QB."
Then the person who hears that relays it to someone else as: "Belichick thinks he can win a Super Bowl without Tom Brady." Okay, BB wasn't the one who said it, but still not outrageous.
This gets relayed to the author as, "Belichick thinks he could have matched his success with any top 15 QB." Whoa now...
Then it gets WRITTEN as, "Belichick thinks he could have won 5 Superbowls with any top 15 QB, BECAUSE Brady isn't actually that good."
This was an example, I don't have any reason to believe this is exactly how it happened, but it's more plausible to me than thinking the Patriots actually had that mindset.