I think the comparison was median annual income and median days of vacation per year. It doesn’t matter too much if the pay is distributed as a single annual lump sum or paid out every hour.
This might not be a problem if there were another part time job that really worked well for getting paid in the other months, but that seems difficult.
Also for reference, I have been working as a teacher for 6 years and make just shy of 43k.
It's a "study" in the sense that a blog by the American Tobacco Association (fictional entity invented for the purpose of this argument) produces "studies" about the "harmlessness of tobacco smoking".