Technically true, but practically this doesn't mean anything. No one uses Clojars on its own, so if an attacker were able to perform a MITM attack, they could

Technically true, but practically this doesn't mean anything. No one uses Clojars on its own, so if an attacker were able to perform a MITM attack, they could inject a spoofed library into the connection to Central even if the library should be fetched from Clojars.