What makes you think he was the one who downvoted you? You could easily have asked him about his technical decisions without being a jerk.
Besides, this was a personal project, probably created without the intention of eventually open sourcing it. If he only ever intended to use MySQL, and portability across RDBMS systems wasn't a goal, then it's fine as it is.
Besides, this was a personal project, probably created without the intention of eventually open sourcing it. If he only ever intended to use MySQL, and portability across RDBMS systems wasn't a goal, then it's fine as it is.