Does lecturing for your PI count as a "guest lecture" for a CV?
I would limit the term "guest lecture" to cases where you were specifically invited to give a lecture to the course, because of your ability to contribute something special that the regular instructor could not do.  (E.g. you have special expertise on a particular topic, or experiences to relate, or notable achievements, etc.)  I wouldn't apply it to simply covering a class (teaching routine material) because the professor is away.

However, you would normally have a separate section on your CV describing your teaching experience, which would include an entry showing that you were a TA for this course.  Under this entry, you could describe your duties, which could include something like "occasional lecturing".