'ee' vs 'i' in English doesn't really denote length, it's a quality distinction around the tenseness/looseness of the tongue. Say "ship" and say "sheep" in English, and feel the difference in what your tongue is doing. Then do it again, holding the vowel in 'ship' longer and making 'sheep' shorter. The words are still recognisable because you aren't changing the important distinction between the vowels. Greek 'η' etc are tense vowels which are closer to "sheep" than "ship".
Are these actual words or just made for english speakers? The course start is very confusing when you have things like φ, φι, χ, χι, ψ, ψι etc. It feels as if these were made just to fit english pronunciation for the letter more, but then you get confused for not understanding the context. More so when the exercise has audio for let's say "sigma" and then you're supposed to know if it's σ or σιγμα (how do you get the ´ for greek btw?). The course makes it quite difficult as non-english speaker to answer the tasks for the pi, pee thing. This course could benefit from some theory and pronunciation descriptions.