It's not secretive, it's just that most people are not very interested in the exact type of Wi-Fi chipset used.

You can see the chipset name (BCM43430) on dmesg:

  $ dmesg | grep brcmfmac
  [   26.488164] brcmfmac: F1 signature read @0x18000000=0x1541a9a6
  [   26.512759] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43430-sdio for chip BCM43430/1
  [   26.513315] usbcore: registered new interface driver brcmfmac
  [   26.539406] brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43430-sdio.raspberrypi,model-zero-w.txt