It’s just a country code like others mention. Also I imagine it doubles as the “Marxist-Leninist” acronym in some people’s eyes given the developers’ ideology.
.ml is the internet country code top-level domain for the country of Mali. According to wikipedia, Mali is giving away .ml domains for free in an affort to promote their IT industry. I suspect that's the reason why some instances chose .ml domains. But that's just a guess from me.