ESXi may have previously been installed on a single disk within the system if the drives were still in a JBOD. There is also technically a difference between what is "supported" and what will work. The term supported is there to indicate that the OS/hardware vendor have tested the configuration and they have certified it to work. Therefore it may not be impossible to complete the install on a SW RAID.
However, I would discourage the practice of attempting to complete this installation on the embedded RAID controller for two reasons:
1) There is a high risk of data loss along with an outage.
2) Cisco and/or VMware will not support your solution as it is specifically ignoring the documentation.
The best thing to do would be to equip the server to run properly with a hardware RAID controller or perhaps SD cards. Finally, you could target a single disk for the installation and leave it as a JBOD.