The software he writes is the capital obviously

From Investopedia: Capitalism is focused on the creation of wealth and ownership of capital and factors of production, whereas a free market system is focused on the exchange of wealth or goods and services.

It's not the free market part that defines it as capitalism but rather the fact he owns the method of production. He owns the code he writes and he owns the business processes around those applications. He privately owns the capital assets (applications and business processes) which make the money.

> Where is capital involved? Capitalism is not the only form of open market economy

Right. Like you don't own anything but still somehow sell it on the open market.

Apparently, "you can't have your cake and eat it, too" is a capitalist prejudice as comrades in the USSR would have ensured us.