This suggests that he is not a native English speaker. They tend to use either form exclusively, depending on where they went to primary school.
Or even where they went to university. For words that can be spelled either way, Oxford University (and Oxford Dictionary) uses -ize and Cambridge uses -ise.
Until World War II, -ize was standard for the UK
, but in the past 65 years the use of -ise has become more common.
Anyway, Satoshi is very careful about hiding his identity, so I expect his choice of -ise or -ize is deliberate and does not reveal any information about him.