I don't that phrase is implying that just a move to a 64-bit architecture allowed for these performance improvements. A more relevant clip of the phrase is:
"The 900MHz 32-bit quad-core ARM Cortex-A7 CPU complex has been replaced by a custom-hardened 1.2GHz 64-bit quad-core ARM Cortex-A53. Combining a 33% increase in clock speed with various architectural enhancements, this provides a 50-60% increase in performance in 32-bit mode versus Raspberry Pi 2..."
So, basically, the 33% increase in CPU speed and "various architectural enhancements" (pipeline changes?) provided for this increase, not (just?) the move from 32-bit to 64-bit.
"The 900MHz 32-bit quad-core ARM Cortex-A7 CPU complex has been replaced by a custom-hardened 1.2GHz 64-bit quad-core ARM Cortex-A53. Combining a 33% increase in clock speed with various architectural enhancements, this provides a 50-60% increase in performance in 32-bit mode versus Raspberry Pi 2..."
So, basically, the 33% increase in CPU speed and "various architectural enhancements" (pipeline changes?) provided for this increase, not (just?) the move from 32-bit to 64-bit.