That bug does not appear to mention the warning message you’ve screenshotted. You should probably file a separate bug detailing your exact issue.
The warning is possibly overbroad, but just because a system has booted under BIOS when booted from install media does not guarantee that it will boot from BIOS from the internal drive. Many older UEFI firmware versions specifically use CSM (BIOS) mode for booting external media which can lead to an unbootable install if we don’t include an ESP; this is the reason for the warning.
It is not “required”, but it is certainly intentional that we install both BIOS and UEFI bootloaders under guided installation.
It is not.