No they don't. I'm not sure where they got that number from, but it's either wrong or being misrepresented (costs of entire infrastructure project confused with cost of lights). $10M is off by 1-3 orders of magnitude, depending on if you're counting installation of just one signal or a complete intersection.

It's conceivable for the installation of traffic lights in a remote place to be an expensive project if there's no power available there, but that's a much larger project than just a single traffic light.

I was thinking of a project that redid a four way intersection.

maybe that includes the software and hardware to run the stoplight, the salaries of the operators and maintenance etc and their admin staff, and the all the analysis to figure out when it should be red/green, all that stuff.

No, it absolutely does not.

Depends where it is. On a military base they can be $100M.

I had in mind a project in my area a few years back where redoing a four way major intersection was quoted like that, not actually one literal stoplight just purchased and delivered.