I remember seeing Borg as a kid, and I can give you in writing that the guy had tons of charisma. In spite of or even because his iceberg personality. Borg had this intimidating aura of invincibility; the type who would stare everybody down, no matter how hard the other guy tried, never cracked or showed any weakness. And he had the looks.
McEnroe vs. Borg was like fire vs. ice.
The very aspect of charisma is tenuous and hard to define so it's no surprise to find polar opposites in thought on the very same person. However, typically, there are two distinct parts, how one carries theirselves, their aura, if you will, and their speaking ability. Borg clearly had the former, but did not really have the latter, and failed as a commentator because of it.