There's no general info on how many respondents were reading ebooks, but the survey results do show that ebook usage is highest among respondents in their 30s (18.5%), followed by their younger cohort (11.3%). The older demographics were much less likely to have read an ebook.
And to be perfectly honest I'm not sure this survey is rigorous enough to be worth taking seriously. The original English language report that I found seems to confuse reading ebooks with using an ereader. In fact, the stats mentioned above about the age groups might actually refer to ereader ownership.
There is an important distinction between using an ereader and reading ebooks because reading devices other than ereaders are much more common in Japan than are ereaders. I can't find any data on tablet ownership in Japan, but over a third of the adult population of Japan has a smartphone, with much of the rest of the population owning a cellphone.
If even a tithe of the adult population reads on their phone then it will throw off the Jiji Press survey.