He's literally using sarcasm to mock the idea of fully switching from GNU/Linux to SystemD/Linux. The "/s" at the end indicates the statement is sarcastic, meaning he does not actually believe this switch will happen, but is sarcastically suggesting it will.

Literally just say out loud to yourself with a sarcastic tone :

So when are we going to fully switch from GNU/Linux to SystemD/Linux?

He's not literally asking when this switch will occur, but rather mocking the notion in a sarcastic tone. Sarcasm involves using words in a mocking or ironic way, often to criticize or make a humorous point, rather than stating the opposite of what one means in a literal sense as a rule of thumb, the sarcastic tone is the key. The implied meaning is the opposite of the implied statement within the sarcastic question, rather than stating the opposite directly. I believe you're confused because he's framing it as a question to obfuscate the meaning and make the sarcasm more subtle.