Use the Disk Utility application to erase the flash drive, as shown below. Use the Finder application to mount the Snow Leopard ISO file.
The Snow Leopard ISO file was the same as discussed in this answer, which should be the same ISO you have linked to in your question. The flash drive needs to be at least 8 GB in size. The procedure should be the same for Catalina. This was tested using an iMac (21.5-inch, Mid 2011) 2.5 GHz Intel Core i5 with MacOS High Sierra 10.13.6 installed. Creating a USB Flash Drive Installer from a Snow Leopard ISO File
