You don't need an ISO. The .bin file is a bootable disk image. Write it directly to USB using Rufus (Windows) or BalenaEtcher (Mac/Win/Linux).
With the ISO file downloaded, you'll need to create a bootable USB drive to install Chrome OS Flex. You can use tools like: Chrome Os Flex Download Iso 64-bit Offline Installer
: Once the USB is created, you can use it to install the OS on target machines without an internet connection during the initial flashing process, though you will need a network later for Google account setup. System Requirements You don't need an ISO
If you have successfully downloaded the .bin recovery image mentioned in Step 1: Chrome Os Flex Download Iso 64-bit Offline Installer