That says ITINERARY, it does not anywhere say you need a ticket. The e-visa application for non-O just asks for flight information in and flight information out. A quick google flights search can get you that information. It's just ticking off a box because a non-O is only valid for 90 days, so you give a flight out within 90 days and done.
What is the game plan then? Is this just a temporary trip to Thailand? Otherwise the only option moving forward is going to be extensions, or returning back to the US every 2 years to apply for a new non-OA visa.
A visa grants you the full stay from the day you arrive. The day you arrive is counted as day 1 and you'll receive 89 additional days after that.
Departure information isn't mandatory for ANYONE on the TDAC. It's not a mandatory field. Don't touch it and it will remain not required. Touch it and it all becomes mandatory.
The advice is don't apply for a non-OA visa. Why are you applying for that? It's only useful in very specific circumstances and is not the proper visa for most people. The non-O is the proper visa for most people and doesn't have ANY of the requirements, just proof that you are over 50 and proof of funds, that's it.
So many reports of people only getting a month METV that applied from Japan. You need to actually do research and use the search feature of this group.