Yes, good explanation. :)
If they included "moikka", "moro", "heipparallaa", "moikkelis koikkelis" and other increasingly more informal and bizarre greetings in one lesson named "informal greetings" or something like that, then "morjes/morjesta" would certainly be there.
"Has no place on a teaching platform" is just overly harsh but I agree with chilipalko's notion. Informal greetings would be a nice bonus vocab collection to add (a lot) later on, possibly. The common greetings should of course be focused on first.
Which is kinda why I'm surprised that they decided to start with terve instead of the much, much more common hei or moi.