You're being that guy. Plus, you're wrong.

Mega- is a standard SI prefix, and the use of "megaton" to denote one million tons is entirely correct. Yes, it's also true that "megaton" has a second meaning. This fact does not invalidate its other meaning. In fact, if you bother looking this up in actual dictionaries, you'll find that the normal mass definition is often listed first, and the explosive definition is listed second, for what it's worth.