Leagues are measurements of linear distance, while degrees are measurements of angles. One can measure distance by calculating difference in degrees of longitude and latitude.
It's not clear to me either, but my understanding is that the text is saying as near as anyone can guess, he traveled 27 leagues, rounded to the nearest whole degree (ignoring minutes and seconds) on a latitude along the equator.
I am not 100% certain that's what it's saying, however.
Student or Learner