In the real world we so far just managed to keep space free of military weapons not because they are expensive, but because of treaties. I just don't expect those to last much longer.
And frankly I never looked into the concept, but why do you think, a space base tungsten penetrator would be way more expensive, than a nuke on a missile?
The idea as proposed in various places is a very large multi-ton vehicle. He chose to focus on trivia. Pretty common for Elon fans.
Humans are pretty clever at devising ways to kill each other. The Russians put a garbage can of concrete on a ersatz ballistic missile on a terror strike against Ukraine. Like you said, these things don’t exist because of treaties.
In the real world we so far just managed to keep space free of military weapons not because they are expensive, but because of treaties. I just don't expect those to last much longer.
And frankly I never looked into the concept, but why do you think, a space base tungsten penetrator would be way more expensive, than a nuke on a missile?