Space launch systems are so deeply dual-use it's not even funny. Orbital rockets are very similar to early, low-readiness ICBMs - modern systems prefer solid fuel, since they can be safely kept fueled for longer periods of time, but aside from that the biggest differences between a Falcon 9 and an ICBM is how ridiculously overpowered the Falcon is, and its lack of re-entry countermeasures (MIRVs, decoys, etc.). Which is why anyone working at a company like SpaceX is subject to the same technology export restrictions as weapons companies.

For example, if SpaceX can get e.g. the Dragon capsule to land with a reasonable accuracy on thrusters, and then put a warhead in it (or multiple warheads) , that's quite a capable weapons system right there. In fact, it's even capable of fractional orbital bombardment, which was considered so destabilizing during the Cold War that it was specifically banned in SALT II.

tl;dr - Elon Musk will, within a couple of years, control a system that can be adapted into an ICBM within a very short period of time and with minimal investment.