Shelby said in her speech that the launch NX-01 lead to 'what we now know as starfleet'.Frontier Day is the anniversary of “Starfleet.” There is nothing at all in the show stating that it has to do with the NX-01’s launch. Unless they’re retconning the founding of Starfleet as 2151 instead of the actual date it was founded.
So she wasn't saying Starfleet was founded with the NX-01's launch, but just lead to the evolution of the modern starfleet.