Plese let me ask...
I know nothing about this technique, I have never germinated cactus seeds, so all is just my assumption.
I believe the peroxide is there to kill everything what could cause fail of germination.
If I was germinating the seeds, I would sterilize the soil and the container, then I would put the soil into the container and moisten the soil with diluted peroxide. Then put the seeds. Then spray by diluted peroxide and close the lid. Everyday I would spray by diluted peroxide, but just the surface with seeds.
I believe there would be little possibility of contamination.
What do you think? Does it make sense or is it complete nonsense?