Yes, we will add "inundation" but a native English speaker would not usually use it as a noun. But you seem to have found it in a dictionary. It's best to appreciate the work done by the team to give you a normal interpretation of the Greek. And we are teaching Greek here and the best way to learn it is to use the hints given.
TIPS TO HELP YOU LEARN 1 Use the Drop-Down hints to help you translate. Pass your cursor over a word and a list of translations will appear. The top word/phrase is the best choice. This will assure that you always have the right translation. It's the secret to success. Do not hesitate to use these as often as you need to.
2 Read the Tips & notes, on the first page of each lesson you’ll see TIPS. Click on that.
3 Always read the comments before posting. Check the heading on the page to see the sentence and its translation. Click on any blue words for more definitions.
4 If your translation is rejected you should carefully compare what you wrote with the answer given. If you do not see a mistake use the Report options at the foot of the exercise page to Report issues such as My answer should be accepted.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM https://forum.duolingo.com/comment/22424028
And check out the Greek Forum here with more links. https://forum.duolingo.com/topic/936