Time isn't the only corporate asset he could have utilized to create his competing code. He might have utilized corporate-funded computers, LAN hardware, Internet access/bandwidth, etc. When I worked for a major Internet corporation I signed a contract specifically stating that anything I created using any corporate resources belonged to the company. Thus, when I worked on open source stuff, I did it from home using my own computer on my own time. Did the author sign such a contract, or violate such a contract by using company assets other than business hours? We just don't know.
Maybe he was breathing some residual company oxygen while he was in his home working on this software. It would follow that he used company resources and thus the software he created must belong entirely to his former employer.
Doesn't he claim this is not true?
"I did not do so on company time (as proven by the git commit logs), I worked on it in the evenings after I had completed my work for the day."