Unfortunately this does matter, and very much so on the Xperia 10 III, because Sony’s Android updates also update the firmwares on the firmware partitions for WLAN, Bluetooth, cellular modem, audio DSP, Camera etc.
But aside of negative effects (e.g. on the Xperia 10 III: “tinted screen”, “echo for callers” etc.), the “Android version that was on the device pre SFOS” cannot be determined after flashing SailfishOS.