I think when another Linux system is booted from grub’s boot menu the other’s system boot.img and core.img are not needed and therefore grub boots the other Linux system at an elevated stage where the differences in booting between UEFI and BIOS do not matter anymore.
In fact, if I click on the Windows 10 entry, that’s all the message is, after which the grub main menu returns after a while. I was inattentive because during the EOS offline installation, during a manual partitioning, I created a new FAT32 small partition for BIOS Grub in response to a warning window. Otherwise, the installer would probably have overwritten the UEFI grub created when installing Kubuntu on the other, original EFI partition.
Actually, many different setups can co-exist on as UEFI partition. A directory is created to hold grub’s start file only, the actual grub info is kept on /boot for that particular system. You can only run in to trouble if UEFI is too small for how many you have installed. For instance I have 8 systems on one computer without crowding UEFI partition (500mb).
If you haven’t checked, it is possible that your BIOS/firmware supports something like Hybrid BIOS (or whatever name with your motherboard vendor, means support for both UEFI and legacy BIOS boot). Worth checking, it might help.
Fwiw, when you do an UEFI installation, the systems installed put their bootloaders in their own folders on ESP. In your case, EnOS would have just added a new folder with its bootloader alongside the other ones already on ESP.
The normal startup has been configured (using autodetect for example) for your system. The Fallback option skips autodetect, and leaves everything (including the kitchen sink) in the startup. It can help with tracking down problems with hardware, or misconfiguration for instance.