Given the rather unpredictable nature of English pronunciation that's basically required for any made up word.
Particularly since it would not be unreasonable to assume that the "mi" in mimalloc is incorrectly pronounced like the "mi" in Microsoft.
That's what happens when languages like English adopt the foreign pronunciation of words instead of the English pronunciation. Now everything is an edge case and you can't be sure of the pronunciation even if it's an English word.
Cool fact - I'd bet that no one pronounces MySQL like its author does.