My comment was based on observations of my SO, who is a high school biology teacher. She legitimately does not have to work at all for the great majority of the summer if she doesn't want to. I understand this may vary a lot by school district and location, and of course even if you don't have to work over the summer there are still benefits to doing so.
The mere fact that many do take summer jobs is proof of that, IMO.
Or it's a proof that they need the money.
Teachers actually need to cram a lot in that time. All the preparation for next year, all self-education they need to do to stay up-to-date, select reading and learning materials. Planning the curriculum etc. The equivalent to all that work that freelancers factor into their daily rate and all the work that employees often do on the job ("I'll need to read the documentation so I know how to properly do that", "I'll google it", ...).
The fact that they still take on summer jobs on top of that can serve as an indicator that they actually don't have too much to do in that time or it can serve as an indicator that they need the money more than their time off.
They have over 20 days of holidays built in during the school year and a legitimate two months off between years.