The video linked in the above post has the guy mentioning the cartridge holder tool at 3:26 and 6:47, shows the tools (he has to try a couple different ones in his tool collection to find the correct size) at 6:54, 7:29, and 7:43, and finally shows the tool in use at 9:40.
I have seen the fork disassembled without this tool, and it's done with an air wrench quickly breaking the bottom bolt loose while the fork is still assembled, the reason being the fork spring tension will help keep the damper rod from spinning while loosening the bottom bolt.
My opinion is to have the tool while doing this job and to use it.
Yamaha motorcycle tools shown in the Yamaha Service Manual are sold to Yamaha motorcycle dealers by K&L Supply Co., and the part number for the damper rod holder is:
YM-01454