His name is Chinese, not Japanese so I don't think he's getting them from Japan. I looked at mine and the one I got from him. Seems to be exactly the same type of moss.
Your right these may not be the actual Marimo Balls from Japan. If you look at the image I attached you can see that these are more furry. The hairs like structure extends outward away from the center. The balls I got from this guy and petsmart are not like this, they are more of a woven strand and do not extend outwards from the center but in any haphazard pattern. Thanks for bringing this to my attention.
The wikki page says they grow about 5mm a year. I think this is probably due to the low level of light in the lake.
Well this just adds some fuel to the fire considering these are not true Marimo Balls. These are definitely falsely advertised.