This is incorrect, that firmware is not included in the installer (for legal reasons, probably). firmware-b43-installer is just a script. It downloads some driver from a website and extracts the firmware blobs from that driver using another utility called b43-fwcutter. The package description for that reads:
Description: utility for extracting Broadcom 43xx firmware
This package provides a tool for extracting BCM43xx wireless chip firmware from Broadcom’s proprietary driver files.
It is used by the firmware-b43(legacy)-installer packages as part of the automated process of downloading and installing firmware.