The situation is not all that different from their other tool for having multiple ISOs on a usb drive though, is it?
You have ISO images that usually you would write onto a CD or write to a USB drive with dd. These cannot boot by themselves if you just put the ISO file as is onto a FAT32 USB drive or similar. But Ventoy has a stage one boot thing for USB which you can then use to select an ISO file from your USB and somehow boot it.
So iVentoy would work the same way. The stage one is from iVentoy, not from the ISO itself. And that stage one does something that makes it possible to boot the ISO that it retrieved. Similar to how the USB based thing somehow can boot the ISO files from the USB.
You have ISO images that usually you would write onto a CD or write to a USB drive with dd. These cannot boot by themselves if you just put the ISO file as is onto a FAT32 USB drive or similar. But Ventoy has a stage one boot thing for USB which you can then use to select an ISO file from your USB and somehow boot it.
So iVentoy would work the same way. The stage one is from iVentoy, not from the ISO itself. And that stage one does something that makes it possible to boot the ISO that it retrieved. Similar to how the USB based thing somehow can boot the ISO files from the USB.