Who knows. The hype over the Carlsbad start-up plans has had more of what turned out to be 'Fake News' than POTUS's tweets.

One report has ADI with options on two Embraer 170s. Not quite 'single-aisle main line equipment' but more in keeping with ADI's existing 145 fleet (if you can have a fleet with 3-5 airframes...).

Also more in keeping with an economical business plan for a start-up. Not that this has held back many a start-up with big-and-shiny-jet-syndrome.

Also, it costs a lot of money to initiate a new aircraft type. Regulatory hurdles, training for pilots & maint, spare parts inventory, and on and on.