It's true that work from a skilled employee is a rival resource, what you pay me to do only benefits you and not the rest of the industry (unless it's open source). But it doesn't seem excludable—if I train employees, there's no mechanism I can use to prevent you from hiring them later if you didn't contribute to the training costs.

Here's where I confess I don't understand why nonrivalry is built into the definition of a public good, since it's nonexcludability that gives rise to the free rider problem.